Fix pid required feature

[tmorrell]

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Description

The fix for the parent doi configuration https://github.com/inveniosoftware/invenio-rdm-records/pull/1740 broke the "required" parameter for the pid provider. Previously you could have a pid provider that was active (shows up in the deposit form), but not required (pid would only be minted if something was entered). Because the check for "required" was removed, this stopped working. A test is also included.

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[tmorrell]

Thanks for the cleanup @utnapischtim I just rebased so this should be good to merge once CI finishes