Fix NIRReflectance passing None as sunz_threshold

[mraspaud]

The documentation of the NIRReflectance compositor says about the sunz_threshold parameter:

Default None, in which case the default threshold defined in Pyspectral will be used.

This wasn't the case, and wasn't tested. This PR fixes the behaviour and adds a test for it.

• Tests added
• Tests passed
• Passes flake8 satpy
• Fully documented

[coveralls]

Coverage increased (+0.02%) to 90.081% when pulling 825a39c on mraspaud:fix-near-refl into 452f8ad on pytroll:master.

[codecov]

Codecov Report

Merging #1319 into master will increase coverage by 0.01%.
The diff coverage is 97.77%.

@@            Coverage Diff             @@
##           master    #1319      +/-   ##
==========================================
+ Coverage   90.06%   90.08%   +0.01%     
==========================================
  Files         220      220              
  Lines       32420    32472      +52     
==========================================
+ Hits        29199    29251      +52     
  Misses       3221     3221              

Impacted Files	Coverage Δ
satpy/composites/__init__.py	81.32% <94.44%> (+0.48%)	⬆️
satpy/tests/test_composites.py	99.89% <100.00%> (+<0.01%)	⬆️

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[adybbroe]

LGTM. Very nice with the refactoring!