menu label should not be nullable

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

with this change, the sandbox menu behaves correct again. i notice that somewhere we use the phpcr node name as fallback if the label is not defined. but i think it makes more sense to have the label required.

the underlying issue is that a while ago, we broke the attribute translation strategy for nullable fields when a translation locale does not exist at all, as all fields then become null:
https://github.com/doctrine/phpcr-odm/blob/master/lib/Doctrine/ODM/PHPCR/Translation/TranslationStrategy/AttributeTranslationStrategy.php#L94

i will try to find a solution for phpcr-odm, but its a very tricky problem.