Allow trends to run even if a component does not have any data due to min_yrs constraints

[heikoklein]

Trends may have a min_yrs constraint rejection all stations which haven't measured at least min_yrs. This can lead to components having 0 data. 0 data crashed pyaerocom (or raised a TrendException). This PR changes the behaviour so that 0 data of a trend-analysis is an acceptable result. It will be logged if the min_yrs constraint removed all stations.

Checklist

• Start with a draft-PR
• The PR title is a good summary of the changes
• PR is set to AeroTools and a tentative milestone
• Documentation reflects the changes where applicable
• Tests for the changes exist where applicable
• Tests pass locally
• Tests pass on CI
• At least 1 reviewer is selected
• Make PR ready to review

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 78.99%. Comparing base (8355b4a) to head (2e9608f).
Report is 313 commits behind head on main-dev.

Files with missing lines	Patch %	Lines
pyaerocom/geodesy.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           main-dev    #1313      +/-   ##
============================================
- Coverage     79.00%   78.99%   -0.01%     
============================================
  Files           136      136              
  Lines         20797    20798       +1     
============================================
  Hits          16430    16430              
- Misses         4367     4368       +1     

Flag	Coverage Δ
unittests	78.99% <83.33%> (-0.01%)	⬇️

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[dulte]

Looks good