fix case of a multiselect dropdown becoming single when no selection

[kecnry]

Description

This pull request fixes turning off multiselect mode for a dropdown when no selections are currently made. Previously this resulted in a traceback, but now applies the default selection for single-select mode (which may either be empty or the first entry, etc, depending on the options for that dropdown).

To reproduce:

• load any viz tool with multiple layers and/or viewers
• open plot options
• select advanced multiselect mode in top right
• in either viewer or layer, clear the selection
• click the button again in the top right to enter single-select mode
• on main, this crashes, but is handled gracefully in this PR (for plot options, the dropdown remains empty)

Note: further improvements to these edge cases could be made (to hide all other plot options information when either viewer or layer is empty, rather than showing empty widgets with "NULL", etc), but those are a larger effort and at least don't result in tracebacks right now.

Change log entry

• Is a change log needed? If yes, is it added to CHANGES.rst? If you want to avoid merge conflicts,
  list the proposed change log here for review and add to CHANGES.rst before merge. If no, maintainer
  should add a no-changelog-entry-needed label.

Checklist for package maintainer(s)

This checklist is meant to remind the package maintainer(s) who will review this pull request of some common things to look for. This list is not exhaustive.

• Are two approvals required? Branch protection rule does not check for the second approval. If a second approval is not necessary, please apply the trivial label.
• Do the proposed changes actually accomplish desired goals? Also manually run the affected example notebooks, if necessary.
• Do the proposed changes follow the STScI Style Guides?
• Are tests added/updated as required? If so, do they follow the STScI Style Guides?
• Are docs added/updated as required? If so, do they follow the STScI Style Guides?
• Did the CI pass? If not, are the failures related?
• Is a milestone set? Set this to bugfix milestone if this is a bug fix and needs to be released ASAP; otherwise, set this to the next major release milestone.
• After merge, any internal documentations need updating (e.g., JIRA, Innerspace)?

[pllim]

LGTM. Thanks!

[codecov]

Codecov Report

Patch coverage is 100.00% of modified lines.

Files Changed	Coverage
jdaviz/core/template_mixin.py	100.00%

📢 Thoughts on this report? Let us know!.

[pllim]

I think the RTD webhook is stuck, blocking auto merge. Since this PR doesn't touch docs, I am just going to merge it.