Fix NPE using CVOC

[luddaniel]

What this PR does / why we need it:

Fixes a possible NullPointerException that prevents the proper indexing of a dataset.

Which issue(s) this PR closes:

Closes #10869

Suggestions on how to test this:

Play around :CVocConf and having multiple configuration activated like SKOMOS + ORCID

Does this PR introduce a user interface change?:

No

Is there a release notes update needed for this change?:

I don't think

[qqmyers]

Makes sense. - Please add a one line release note, e.g. - This release fixes a bug in the external controlled vocabulary mechanism (introduced in v6.3) that could cause indexing to fail when a script is configured for one child field and no other child fields were managed.

FWIW: I didn't see this error using ORCID and ROR for the author field (requiring #10712) because the NPE is avoided due to the ORCID script registering to manage the other fields. - not sure if that provides a way to work-around in 6.3 or not, i.e. by adding a fake registration for a second (empty, missing?) script that would include the managed fields.