Fix nullability of Equals methods.

src/Markup/Avalonia.Markup/Markup/Parsers/PropertyPathGrammar.cs

@@ -188,7 +188,7 @@ namespace Avalonia.Markup.Parsers
         {
             public string Name { get; set; } = string.Empty;
 
-            public override bool Equals(object obj)
+            public override bool Equals(object? obj)
                 => obj is PropertySyntax other
                    && other.Name == Name;
         }
@@ -199,7 +199,7 @@ namespace Avalonia.Markup.Parsers
             public string TypeName { get; set; } = string.Empty;
             public string? TypeNamespace { get; set; }
 
-            public override bool Equals(object obj)
+            public override bool Equals(object? obj)
                 => obj is TypeQualifiedPropertySyntax other
                    && other.Name == Name
                    && other.TypeName == TypeName
@@ -209,14 +209,14 @@ namespace Avalonia.Markup.Parsers
         public class ChildTraversalSyntax : ISyntax
         {
             public static ChildTraversalSyntax Instance { get;  } = new ChildTraversalSyntax();
-            public override bool Equals(object obj) => obj is ChildTraversalSyntax;
+            public override bool Equals(object? obj) => obj is ChildTraversalSyntax;
         }
         
         public class EnsureTypeSyntax : ISyntax
         {
             public string TypeName { get; set; } = string.Empty;
             public string? TypeNamespace { get; set; }
-            public override bool Equals(object obj)
+            public override bool Equals(object? obj)
                 => obj is EnsureTypeSyntax other
                    && other.TypeName == TypeName
                    && other.TypeNamespace == TypeNamespace;
@@ -226,7 +226,7 @@ namespace Avalonia.Markup.Parsers
         {
             public string TypeName { get; set; } = string.Empty;
             public string? TypeNamespace { get; set; }
-            public override bool Equals(object obj)
+            public override bool Equals(object? obj)
                 => obj is CastTypeSyntax other
                    && other.TypeName == TypeName
                    && other.TypeNamespace == TypeNamespace;

src/Markup/Avalonia.Markup/Markup/Parsers/SelectorGrammar.cs

@@ -331,7 +331,7 @@ namespace Avalonia.Markup.Parsers
 
             public string Xmlns { get; set; } = string.Empty;
 
-            public override bool Equals(object obj)
+            public override bool Equals(object? obj)
             {
                 var other = obj as OfTypeSyntax;
                 return other != null && other.TypeName == TypeName && other.Xmlns == Xmlns;
@@ -344,7 +344,7 @@ namespace Avalonia.Markup.Parsers
 
             public string Xmlns { get; set; } = string.Empty;
 
-            public override bool Equals(object obj)
+            public override bool Equals(object? obj)
             {
                 var other = obj as IsSyntax;
                 return other != null && other.TypeName == TypeName && other.Xmlns == Xmlns;
@@ -355,7 +355,7 @@ namespace Avalonia.Markup.Parsers
         {
             public string Class { get; set; } = string.Empty;
 
-            public override bool Equals(object obj)
+            public override bool Equals(object? obj)
             {
                 return obj is ClassSyntax && ((ClassSyntax)obj).Class == Class;
             }
@@ -365,7 +365,7 @@ namespace Avalonia.Markup.Parsers
         {
             public string Name { get; set; } = string.Empty;
 
-            public override bool Equals(object obj)
+            public override bool Equals(object? obj)
             {
                 return obj is NameSyntax && ((NameSyntax)obj).Name == Name;
             }
@@ -377,7 +377,7 @@ namespace Avalonia.Markup.Parsers
 
             public string Value { get; set; } = string.Empty;
 
-            public override bool Equals(object obj)
+            public override bool Equals(object? obj)
             {
                 return obj is PropertySyntax && 
                     ((PropertySyntax)obj).Property == Property && 
@@ -387,7 +387,7 @@ namespace Avalonia.Markup.Parsers
 
         public class ChildSyntax : ISyntax
         {
-            public override bool Equals(object obj)
+            public override bool Equals(object? obj)
             {
                 return obj is ChildSyntax;
             }
@@ -395,7 +395,7 @@ namespace Avalonia.Markup.Parsers
 
         public class DescendantSyntax : ISyntax
         {
-            public override bool Equals(object obj)
+            public override bool Equals(object? obj)
             {
                 return obj is DescendantSyntax;
             }
@@ -403,7 +403,7 @@ namespace Avalonia.Markup.Parsers
 
         public class TemplateSyntax : ISyntax
         {
-            public override bool Equals(object obj)
+            public override bool Equals(object? obj)
             {
                 return obj is TemplateSyntax;
             }
@@ -413,7 +413,7 @@ namespace Avalonia.Markup.Parsers
         {
             public IEnumerable<ISyntax> Argument { get; set; } = Enumerable.Empty<ISyntax>();
 
-            public override bool Equals(object obj)
+            public override bool Equals(object? obj)
             {
                 return (obj is NotSyntax not) && Argument.SequenceEqual(not.Argument);
             }
@@ -421,7 +421,7 @@ namespace Avalonia.Markup.Parsers
 
         public class CommaSyntax : ISyntax
         {
-            public override bool Equals(object obj)
+            public override bool Equals(object? obj)
             {
                 return obj is CommaSyntax or;
             }