Fix regression from valpropertymsg directive changes

[nathanwoulfe]

Fixes regression where validation object doesn't always exist on the property, depending on context - when adding an editor to a content type, the validation logic runs, but the validation property on the editor doesn't exist.

Changes here add an explicit check for currentProperty.validation before trying to access .mandatory

This relates to the changes from #7662 - ping @poornimanayar since she's across those changes 😄

Prerequisites

• I have added steps to test this contribution in the description below

To test, edit a content type, adding a new property editor, should be no JS errors logged into the console. Validation should clear as expected when editing the property value on a content item.

[nathanwoulfe]

Try adding a text box to an existing type, before applying this change - will throw a js error when the editor config overlay opens, because validation isn't a property on the property editor. If there are other places where the property editor model doesn't include the validation object, this fix will manage those too

[poornimanayar]

Well done spotting it @nathanwoulfe. This totally works, happy to approve. And thanks for adding the extra notes for testing :-)

[poornimanayar]

And merged your work ⭐️ ! Thanks as always :-)

[nathanwoulfe]

No worries - happened to have Dev tools open and noticed the error. I don't think it actually broke anything since the model value never changed so was never validated, but still needed to be fixed