Fix 500 errors when uploading single controlled vocab or compound field values to editMetadata API

[vera]

What this PR does / why we need it:

This PR fixes a 500 server error returned when updating two specific kinds of fields using the /api/datasets/<id>/editMetadata API:

• a single/non-multiple controlled vocab value (e.g. journalArticleType in the journals block)
• a value inside a single/non-multiple compound field (e.g. targetSampleSizeFormula in targetSampleSize in the socialscience block)

Which issue(s) this PR closes:

• Closes Using editMetadata API endpoint fails when editing non-multiple compound or vocabulary fields #8860

Special notes for your reviewer:

/

Suggestions on how to test this:

I've added tests for the two cases that can be run with mvn test -Dtest="DatasetsIT#testAddUpdateDatasetViaNativeAPI"

Does this PR introduce a user interface change? If mockups are available, please link/include them here:

/

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

/

Additional documentation:

/

[coveralls]

coverage: 22.75% (-0.001%) from 22.751%
when pulling 69b9a01 on vera:edit-metadata-api-500-error
into 69ebed2 on IQSS:develop.

[sekmiller]

Looks good. Nice catch. Thanks!