Add missing default=None

[twoodwark]

On issue #1027 there was a problem with CLI option defaults overriding pyproject.toml. Here's another one, use-default-kwarg

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (e9b6edf) 100.00% compared to head (d6fd3ae) 100.00%.
Report is 705 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##            master     #1515     +/-   ##
===========================================
  Coverage   100.00%   100.00%             
===========================================
  Files           11        32     +21     
  Lines         1020      3589   +2569     
  Branches       201       840    +639     
===========================================
+ Hits          1020      3589   +2569     

Flag	Coverage Δ
unittests	99.63% <99.58%> (?)

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

Files Changed	Coverage Δ
datamodel_code_generator/__init__.py	100.00% <100.00%> (ø)
datamodel_code_generator/__main__.py	100.00% <100.00%> (ø)
datamodel_code_generator/format.py	100.00% <100.00%> (ø)
datamodel_code_generator/http.py	100.00% <100.00%> (ø)
datamodel_code_generator/imports.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/__init__.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/base.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/dataclass.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/enum.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/imports.py	100.00% <100.00%> (ø)
... and 22 more

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