subset plugin: disable update button when no subset selected

[kecnry]

Description

This pull request disables the "update" button when no subset selected.

Screen.Recording.2024-05-20.at.2.45.19.PM.mov

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)?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.72%. Comparing base (3ac9bfb) to head (8a7174a).
Report is 157 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2880   +/-   ##
=======================================
  Coverage   88.72%   88.72%           
=======================================
  Files         111      111           
  Lines       17102    17102           
=======================================
  Hits        15174    15174           
  Misses       1928     1928           

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[rosteen]

Simple enough!