fix: handle single file i18n correctly

[KaneFreeman]

Related to decaporg/decap-cms#6937, decaporg/decap-cms#6898 and #935

Fixes bug caused by #868

Fix in decap: decaporg/decap-cms#6942

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[codecov]

Codecov Report

Merging #936 (58391d5) into main (cc07c77) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #936      +/-   ##
==========================================
- Coverage   52.27%   52.26%   -0.02%     
==========================================
  Files         230      230              
  Lines        9880     9882       +2     
  Branches     2584     2585       +1     
==========================================
  Hits         5165     5165              
- Misses       4331     4332       +1     
- Partials      384      385       +1     

Files	Coverage Δ
packages/core/src/backend.ts	12.37% <0.00%> (-0.05%)	⬇️