support deleting all viewers without crashing app

[kecnry]

Description

This pull request "fixes" the viewer select to allow it to become empty if all viewers are removed from the app. It also hides elements in the plot options plugin that are not applicable if no viewers are selected.

None of the configs in jdaviz currently allow deleting all viewers, so this should not affect users in any way.

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. Bugfix milestone also needs an accompanying backport label.
• After merge, any internal documentations need updating (e.g., JIRA, Innerspace)?

[pllim]

As long as Imviz keeps the default viewer always, this is fine. Thanks!

[kecnry]

As long as Imviz keeps the default viewer always, this is fine.

I see no reason we can't allow that in the future, but there are currently a bunch of assumptions that will need to be removed, so this does not aim to support that (yet).