[babel] Fix, crash with empty LANGUAGES config key

[dpgaspar]

Fixed:

• Empty LANGUAGES definition crash

• Remove drop down language selection if LANGUAGES is empty or just one

• Fix API requesting a non existing language crash

[codecov]

Codecov Report

Merging #1147 into master will increase coverage by 0.22%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1147      +/-   ##
==========================================
+ Coverage   68.83%   69.05%   +0.22%     
==========================================
  Files          45       45              
  Lines        6248     6254       +6     
==========================================
+ Hits         4301     4319      +18     
+ Misses       1947     1935      -12

Impacted Files	Coverage Δ
flask_appbuilder/babel/manager.py	95.23% <100%> (+3.57%)	⬆️
flask_appbuilder/fieldwidgets.py	62.83% <0%> (+6.19%)	⬆️
flask_appbuilder/babel/views.py	100% <0%> (+36.36%)	⬆️

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