fixes Should_Return_TargetNullValue_When_Value_Is_Null fails

src/Markup/Avalonia.Markup/Data/MultiBinding.cs

@@ -48,7 +48,7 @@ namespace Avalonia.Data
         /// Gets or sets the binding priority.
         /// </summary>
         public BindingPriority Priority { get; set; }
-        
+
         /// <summary>
         /// Gets or sets the relative source for the binding.
         /// </summary>
@@ -77,12 +77,12 @@ namespace Avalonia.Data
             // We only respect `StringFormat` if the type of the property we're assigning to will
             // accept a string. Note that this is slightly different to WPF in that WPF only applies
             // `StringFormat` for target type `string` (not `object`).
-            if (!string.IsNullOrWhiteSpace(StringFormat) && 
+            if (!string.IsNullOrWhiteSpace(StringFormat) &&
                 (targetType == typeof(string) || targetType == typeof(object)))
             {
                 converter = new StringFormatMultiValueConverter(StringFormat, converter);
             }
-            
+
             var children = Bindings.Select(x => x.Initiate(target, null));
 
             var input = children.Select(x => x.Observable)
@@ -116,8 +116,13 @@ namespace Avalonia.Data
             }
 
             var culture = CultureInfo.CurrentCulture;
-            var converted = converter?.Convert(values, targetType, ConverterParameter, culture)
-                ?? values.ToArray();
+            object converted;
+            if (converter != null)
+            {
+                converted = converter.Convert(values, targetType, ConverterParameter, culture);
+            }
+            else
+                converted = values.ToArray();
 
             if (converted == null)
             {