V8: Mandatory check-box list isn't validated

[AndyButland]

To replicate:

• Create a data type based on the "Checkbox list"
• Add a few options
• Create a property on a document type using the new data type
• Make it mandatory
• Try entering content based on the document type
• Whether or not a check box is checked, the property isn't marked as invalid. I'd expect it should be if no value is checked.

This does look like a bug rather than something not implemented for this property editor, as the mandatory property is referenced in the checkboxlist.html file.

[kjac]

I believe this is a simple view error; I'll submit a fix shortly.

Rather unfortunate that the property editor isn't validated server side, though... 😨

[kjac]

PR in #5780

[nul800sebastiaan]

Fixed in #5780
Cherry picked for 8.1.5 - 9c12b4e