CircleCI: Handle null vcs.provider_name

[jwodder]

Closes #189.

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 61.29%. Comparing base (9ca71bc) to head (97fcd40).

Files	Patch %	Lines
src/tinuous/circleci.py	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #190      +/-   ##
==========================================
+ Coverage   61.24%   61.29%   +0.05%     
==========================================
  Files          10       10              
  Lines        1370     1372       +2     
  Branches      284      284              
==========================================
+ Hits          839      841       +2     
- Misses        457      459       +2     
+ Partials       74       72       -2     

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[yarikoptic]

Great - thanks! we had no release for awhile, let's do it!