Improve compatibility with pluralized strings referenced as singulars

[swissspidy]

Handle cases where a pluralized string is only used as a singular one.

For example, when both __( 'Product' ) and _n( 'Product', 'Products' ) are used, the entry key will have the format "ProductNULProducts". Fall back to looking up just "Product" to support this edge case.

[codecov]

Codecov Report

Attention: 11 lines in your changes are missing coverage. Please review.

Comparison is base (efa850e) 92.50% compared to head (85b58a2) 90.60%.

Files	Patch %	Lines
lib/class-ginger-mo-translation-file.php	15.38%	11 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #151      +/-   ##
============================================
- Coverage     92.50%   90.60%   -1.90%     
- Complexity      221      223       +2     
============================================
  Files             6        6              
  Lines           520      532      +12     
============================================
+ Hits            481      482       +1     
- Misses           39       50      +11     

Flag	Coverage Δ
default	53.38% <15.38%> (-1.04%)	⬇️
integration	39.66% <0.00%> (-0.92%)	⬇️
multisite	39.66% <0.00%> (-0.92%)	⬇️
trunk	65.41% <15.38%> (-1.32%)	⬇️
unit	53.38% <15.38%> (-1.04%)	⬇️

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