Models: PropertyType constructor sets the DataTypeKey if IDataType has identity (#20301)

* PropertyType constructor sets the DataTypeKey if passed IDataType has identity

* Updated unit tests to verify behaviour.

---------

Co-authored-by: Andy Butland <abutland73@gmail.com>

src/Umbraco.Core/Models/IPropertyType.cs

@@ -24,6 +24,9 @@ public interface IPropertyType : IEntity, IRememberBeingDirty
     /// </summary>
     int DataTypeId { get; set; }
 
+    /// <summary>
+    ///     Gets or sets the Guid unique identifier of the datatype for this property type.
+    /// </summary>
     Guid DataTypeKey { get; set; }
 
     /// <summary>

src/Umbraco.Core/Models/PropertyType.cs

@@ -47,6 +47,7 @@ public class PropertyType : EntityBase, IPropertyType, IEquatable<PropertyType>
         if (dataType.HasIdentity)
         {
             _dataTypeId = dataType.Id;
+            _dataTypeKey = dataType.Key;
         }
 
         _propertyEditorAlias = dataType.EditorAlias;
@@ -159,6 +160,7 @@ public class PropertyType : EntityBase, IPropertyType, IEquatable<PropertyType>
         set => SetPropertyValueAndDetectChanges(value, ref _dataTypeId, nameof(DataTypeId));
     }
 
+    /// <inheritdoc />
     [DataMember]
     public Guid DataTypeKey
     {