Disable spellcheck when language list is empty

[darkdh]

fix #11739

Auditors: @bbondy, @bsclifton

Test Plan:
a. Covered by unittest
b. Manuall test:

1. Make spellchecker lang lists empty
2. Type some random word on about:styles
3. There shouldn't be any red underline

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.

Test Plan:

Reviewer Checklist:

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[codecov-io]

Codecov Report

Merging #11758 into master will decrease coverage by 0.01%.
The diff coverage is 42.85%.

@@            Coverage Diff             @@
##           master   #11758      +/-   ##
==========================================
- Coverage   52.75%   52.74%   -0.02%     
==========================================
  Files         269      269              
  Lines       25668    25670       +2     
  Branches     4097     4099       +2     
==========================================
- Hits        13542    13540       -2     
- Misses      12126    12130       +4

Flag	Coverage Δ
#unittest	52.74% <42.85%> (-0.02%)	⬇️

Impacted Files	Coverage Δ
js/about/preferences.js	46.21% <0%> (-0.37%)	⬇️
app/browser/reducers/spellCheckerReducer.js	78.46% <100%> (-0.65%)	⬇️

[bsclifton]

Manually tested, works great 😄 Includes tests, test plan looks good. Nice fix! ❤️

[bsclifton]

master 4cd4d20
0.21.x c96591a
0.20.x 13e6e16
0.19.x e3a3043