Allow omitting optional arguments with serialize_variables=True

[leszekhanusz]

Fixes #333

[codecov-commenter]

Codecov Report

Merging #338 (2102a27) into master (5912f8f) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #338   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           26        26           
  Lines         2232      2232           
=========================================
  Hits          2232      2232           

Impacted Files	Coverage Δ
gql/utilities/serialize_variable_values.py	100.00% <ø> (ø)

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