Fixed missing null check in BlockEditorPropertyEditor (#11184)

Thanks @lauraneto - reviewed this and your fix solves the issue. #h5yr

(cherry picked from commit 23d2106)

# Conflicts:
#	src/Umbraco.Web/PropertyEditors/BlockEditorPropertyEditor.cs

src/Umbraco.Web/PropertyEditors/BlockEditorPropertyEditor.cs

@@ -258,7 +258,7 @@ public IEnumerable<ValidationResult> Validate(object value, string valueType, ob
                         _textService.Localize("validation", "entriesShort", new[]
                         {
                             validationLimit.Min.ToString(),
-                            (validationLimit.Min - blockEditorData.Layout.Count()).ToString()
+                            (validationLimit.Min - (blockEditorData?.Layout.Count() ?? 0)).ToString()
                         }),
                         new[] { "minCount" });
                 }