Fixes save of empty, invariant block list on variant content. (#18932)

Author: AndyButland

src/Umbraco.Infrastructure/PropertyEditors/BlockValuePropertyValueEditorBase.cs

@@ -296,6 +296,10 @@ private void MapBlockItemDataFromEditor(List<BlockItemData> items)
 
         TValue? mergedBlockValue =
             MergeVariantInvariantPropertyValueTyped(source, target, canUpdateInvariantData, allowedCultures);
+        if (mergedBlockValue is null)
+        {
+            return null;
+        }
 
         return _jsonSerializer.Serialize(mergedBlockValue);
     }

tests/Umbraco.Tests.UnitTests/Umbraco.Core/PropertyEditors/BlockListEditorPropertyValueEditorTests.cs

@@ -75,6 +75,14 @@ public void Validates_Number_Of_Items_Is_Less_Than_Or_Equal_To_Configured_Max(in
         }
     }
 
+    [Test]
+    public void MergeVariantInvariantPropertyValue_Can_Merge_Null_Values()
+    {
+        var editor = CreateValueEditor();
+        var result = editor.MergeVariantInvariantPropertyValue(null, null, true, ["en-US"]);
+        Assert.IsNull(result);
+    }
+
     private static JsonObject CreateBlocksJson(int numberOfBlocks)
     {
         var layoutItems = new JsonArray();