Make getFieldNames correctly defines array of primitives

[Ariqun]

Reasons for making this change

I have custom multiple select that has value like string[]. If i use omitExtraData and liveOmit and chose value, rjsf clear select and rerender it. I have determined that this is due to getFieldNames skipping arrays with primitives ['value1'], causing such fields to be defined as empty.

Checklist

• I'm updating documentation
  • I've checked the rendering of the Markdown text I've added
• I'm adding or updating code
  • I've added and/or updated tests. I've run npm run test:update to update snapshots, if needed.
  • I've updated docs if needed
  • I've updated the changelog with a description of the PR
• I'm adding a new feature
  • I've updated the playground with an example use of the feature