Fix fill value removal for non-integer FCI data

[gerritholl]

Fix removing the _FillValue attribute for non-integer FCI data. Satpy was removing the non-existing FillValue attribute instead. Retaining the _FillValue was causing problems as described in #3058.

• Closes Writing and reading NetCDF file for FCI does not retain fill value #3058
• Tests added

[ameraner]

LGTM, thanks for the fix! I wouldn't be surprised if FillValue was used in an old test file format version available when that part of code was written, and it got fixed in the meanwhile.

[gerritholl]

There are two more occurrences of FillValue in this file and 8 more in the readers for AGRI, FY4B, GHI, MERSI, and MWS...

[ameraner]

Yes, I also noticed. Those could also be fixed to _FillValue (or the according checks could be removed alltogether since the format now is populated and should never trigger them anymore...). Up to you of course if you want to include it in this PR.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.11%. Comparing base (f7d5dba) to head (bbbcb2f).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3060   +/-   ##
=======================================
  Coverage   96.11%   96.11%           
=======================================
  Files         383      383           
  Lines       55685    55694    +9     
=======================================
+ Hits        53523    53532    +9     
  Misses       2162     2162           

Flag	Coverage Δ
behaviourtests	3.88% <0.00%> (-0.01%)	⬇️
unittests	96.21% <100.00%> (+<0.01%)	⬆️

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