cuba.accessGroupsStorageMode property management bug

[michele-tramonti]

Environment

• Platform version: 7.2.16

Description of the bug or enhancement

I think this is a bug:

if in app.properties is specified the property cuba.accessGroupsStorageMode without a defined value, then the screen for groups definition and browse is broken:
you can open it, but show no groups (the same for the lookup field in user edit and create, making creation of new user impossible)