Fix GRIB field when typeOfLevel key missing

[sandorkertesz]

Description

Fixed issue when could not load GRIB data into a fieldlist when the typeOfLevel GRIB key was missing.

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.49%. Comparing base (4283cc0) to head (ff9af36).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #983   +/-   ##
========================================
  Coverage    81.49%   81.49%           
========================================
  Files          235      235           
  Lines        15275    15275           
  Branches       751      751           
========================================
  Hits         12448    12448           
  Misses        2602     2602           
  Partials       225      225           

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