Don't crash if retrieving a corpora with a blank language code

[johnml1135]

Further fix for #595

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This change is 

[Enkidu93]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @johnml1135)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 64.48%. Comparing base (f45b102) to head (f288bdb).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
.../Serval.DataFiles/Controllers/CorporaController.cs	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #600   +/-   ##
=======================================
  Coverage   64.48%   64.48%           
=======================================
  Files         280      280           
  Lines       14105    14105           
  Branches     1825     1825           
=======================================
  Hits         9095     9095           
  Misses       4349     4349           
  Partials      661      661           

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