BUG: Line list to not crash when no data is loaded

[pllim]

Description

This pull request is to fix #1669.

I did not test various possible line list workflows but I did confirm the added test was failing before and not fail anymore with this patch.

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.

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[kecnry]

The plugin no longer crashes when loading the list, but this just seems to kick the can down the road a bit...

• loading and plotting a line will not show (maybe this makes sense). But then loading a spectrum still does not show the line and the user has to toggle it off and back on again.
• the custom line section relies on the data spectral units to set the default units, so that is now defaulting to dimensionless, which crashes when trying to add a custom line.

The alternative would be to disable the entire plugin until data is loaded.

[codecov]

Codecov Report

Base: 87.16% // Head: 87.16% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (20048bf) compared to base (970a915).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1690   +/-   ##
=======================================
  Coverage   87.16%   87.16%           
=======================================
  Files          95       95           
  Lines        9907     9911    +4     
=======================================
+ Hits         8635     8639    +4     
  Misses       1272     1272           

Impacted Files	Coverage Δ
...z/configs/default/plugins/line_lists/line_lists.py	74.56% <100.00%> (+0.22%)	⬆️

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[pllim]

Ooops, ok. Let me open another PR with your other idea. Thanks!

[pllim]

See #1691 for an alternative. Thanks!