Use NULL for zero-argument PyObject_CallMethod format

[mdickinson]

This PR is a minor refactor changing the format for PyObject_CallMethod calls with no arguments from "()" to NULL.

Fixes #1099.

May conflict with #1086. I'll hold off merging this until after #1086 is merged.

[mdickinson]

Drive-by spacing consistency nitpicks.

[codecov-commenter]

Codecov Report

Merging #1100 into master will decrease coverage by 2.32%.
The diff coverage is 66.40%.

@@            Coverage Diff             @@
##           master    #1100      +/-   ##
==========================================
- Coverage   76.15%   73.83%   -2.33%     
==========================================
  Files          54       75      +21     
  Lines        6493     8357    +1864     
  Branches     1263     1592     +329     
==========================================
+ Hits         4945     6170    +1225     
- Misses       1205     1807     +602     
- Partials      343      380      +37     

Impacted Files	Coverage Δ
traits/api.py	100.00% <ø> (+9.67%)	⬆️
traits/base_trait_handler.py	61.76% <ø> (ø)
traits/has_traits.py	72.40% <ø> (-0.37%)	⬇️
traits/observers/_i_notifier.py	0.00% <0.00%> (ø)
traits/observers/events.py	0.00% <0.00%> (ø)
traits/traits.py	75.10% <ø> (-2.45%)	⬇️
traits/util/resource.py	15.25% <ø> (ø)
traits/observers/_generated_parser.py	51.96% <51.96%> (ø)
traits/trait_types.py	72.29% <80.00%> (-0.16%)	⬇️
traits/observers/parsing.py	95.34% <95.34%> (ø)
... and 51 more

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[kitchoi]

LGTM