When no fill value is used, write TransparentPixel=-1 in ninjogeotiff headers

[gerritholl]

When an image with an alpha layer is written, fill value is not applicable. In the satpy source code, this is signalled by fill_value=None. However, this value None should not be written to the TransparentPixel tag. Instead, the standard calls for fill_value=-1 when a fill value is not applicable. This PR makes sure that when an image with an alpha layer is written, the GDALMetaData tag TransparentPixel will be written with the value of -1 by the ninjogeotiff writer.

• Closes ninjogeotiff writer produces file with ninjo_TransparentPixel=None #1930
• Tests added
• Fully documented

[codecov]

Codecov Report

Merging #1931 (1fa096f) into main (a430f86) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1931   +/-   ##
=======================================
  Coverage   93.48%   93.48%           
=======================================
  Files         275      275           
  Lines       40741    40749    +8     
=======================================
+ Hits        38088    38096    +8     
  Misses       2653     2653           

Flag	Coverage Δ
behaviourtests	4.83% <0.00%> (-0.01%)	⬇️
unittests	94.03% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
satpy/tests/writer_tests/test_ninjogeotiff.py	97.70% <100.00%> (+0.04%)	⬆️
satpy/writers/ninjogeotiff.py	99.19% <100.00%> (+0.01%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a430f86...1fa096f. Read the comment docs.

[coveralls]

Coverage increased (+0.001%) to 93.957% when pulling 1fa096f on gerritholl:ninjogeotiff-transparentpixel-none into a430f86 on pytroll:main.

[mraspaud]

LGTM