ui/taxonomy: CustomSelect isClearable - handle empty string selection

[ssangervasi]

Closes #1504

Code Changes

• Use isClearable during onChange in forms.
• Apply this to taxonomy Data Use

Steps to Confirm

• Video of clearing multiple.

Pre-Merge Checklist

• All CI Pipelines Succeeded
• Documentation Updated:
  • documentation complete, or draft/outline provided (tag docs-team to complete/review on this branch)
  • documentation issue created (tag docs-team to complete issue separately)
• Issue Requirements are Met
• Relevant Follow-Up Issues Created
• Update CHANGELOG.md

Description Of Changes

The single-select clearing functionality already existed, but we hadn't tried it anywhere. The only thing it was missing was assigning the value to a "cleared" value. react-select doesn't want to use null or undefined for this state, which is reasonable since the native HTML way of doing it is to set an empty string (whenever the value would not match one of the option elements).

This empty-string case then needs to be handled when transforming the form into an object to submit to the server.

I looked for other uses of CustomSelect where the value should be clearable, but they all seemed required to me. For example, submitting a null/undefined/empty-string data_qualifier is not accepted by the server.

[LKCSmith]

👍