Caused by {rB31365c6b9e4c}.

The new API `CustomDataAttributeProvider` just did not support
`CD_MASK_PROP_STRING`.

While strings dont perform nicely in their current form, still add
support back for the API.

Adding Strings to the supported types seems to survive just
fine, see attached example file.

Maniphest Tasks: T100747

Differential Revision: https://developer.blender.org/D15851