fix: Check for default value presence for non-optional fields in REST

[vam-google]

Otherwise on practical cases we may get 400 errors because of empty body of the non-optional query params.

[codecov]

Codecov Report

Merging #868 (d80ae73) into master (7c185e8) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #868   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           26        27    +1     
  Lines         1608      1694   +86     
  Branches       328       347   +19     
=========================================
+ Hits          1608      1694   +86     

Impacted Files	Coverage Δ
gapic/samplegen_utils/types.py	100.00% <ø> (ø)
gapic/samplegen_utils/utils.py	100.00% <ø> (ø)
gapic/utils/case.py	100.00% <ø> (ø)
gapic/utils/reserved_names.py	100.00% <ø> (ø)
gapic/generator/generator.py	100.00% <100.00%> (ø)
gapic/samplegen/samplegen.py	100.00% <100.00%> (ø)
gapic/schema/api.py	100.00% <100.00%> (ø)
gapic/schema/metadata.py	100.00% <100.00%> (ø)
gapic/schema/wrappers.py	100.00% <100.00%> (ø)
gapic/utils/__init__.py	100.00% <100.00%> (ø)
... and 3 more

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