The file-pattern source does not return the expected object

[sandorkertesz]

This PR fixes the issue when the file-pattern source used with the hive_partitioning=False option did not return the expected data object (e.g. for GRIB data did not return a FieldList).

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.80%. Comparing base (66cddef) to head (89daf9f).
Report is 55 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #697      +/-   ##
==========================================
+ Coverage   90.02%   90.80%   +0.77%     
==========================================
  Files         158      164       +6     
  Lines       12086    12453     +367     
  Branches      593      609      +16     
==========================================
+ Hits        10881    11308     +427     
+ Misses       1023      964      -59     
+ Partials      182      181       -1     

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