corrected heatmap data when entire season is full of NaNs

[jgriesfeller]

Correct #658
The heatmap data is basically computed using the same loop as the map data. The fix just moves the resetting of use_dummy into the season loop.

[codecov]

Codecov Report

Merging #661 (391b9f7) into main-dev (e8f0c45) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##           main-dev     #661   +/-   ##
=========================================
  Coverage     76.90%   76.90%           
=========================================
  Files            98       98           
  Lines         17647    17647           
=========================================
  Hits          13571    13571           
  Misses         4076     4076           

Flag	Coverage Δ
unittests	76.90% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pyaerocom/aeroval/coldatatojson_helpers.py	71.86% <100.00%> (ø)

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[avaldebe]

🚀 ready to merge 🎆