Merge pull request #11904 from MrJul/fixes/datagridcolumn-classes

Fix DataGridColumn.CellStyleClasses not settable from XAML

src/Avalonia.Controls.DataGrid/DataGridColumn.cs

@@ -38,7 +38,7 @@ namespace Avalonia.Controls
         private ICellEditBinding _editBinding;
         private IBinding _clipboardContentBinding;
         private ControlTheme _cellTheme;
-        private readonly Classes _cellStyleClasses = new Classes();
+        private Classes _cellStyleClasses;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="T:Avalonia.Controls.DataGridColumn" /> class.
@@ -393,17 +393,7 @@ namespace Avalonia.Controls
             }
         }
 
-        public Classes CellStyleClasses
-        {
-            get => _cellStyleClasses;
-            set
-            {
-                if(_cellStyleClasses != value)
-                {
-                    _cellStyleClasses.Replace(value);
-                }
-            }
-        }
+        public Classes CellStyleClasses => _cellStyleClasses ??= new();
 
         /// <summary>
         ///    Backing field for CellTheme property.