Handle nested nullable enums

[philipbjorge]

This is a fix for #424

Checked

• ./scripts/test.sh
• ./scripts/format.sh
• ./scripts/lint.sh (I am getting some errors here but no files related to this change)

datamodel_code_generator/format.py:54: error: Module has no attribute "__version__"
datamodel_code_generator/format.py:57: error: Module has no attribute "Config"
datamodel_code_generator/format.py:67: error: Module has no attribute "__version__"
datamodel_code_generator/format.py:70: error: Module has no attribute "SortImports"
datamodel_code_generator/format.py:77: error: Module has no attribute "code"

[codecov]

Codecov Report

Merging #425 (bc4c669) into master (e9b6edf) will not change coverage.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##            master      #425     +/-   ##
===========================================
  Coverage   100.00%   100.00%             
===========================================
  Files           11        20      +9     
  Lines         1020      2131   +1111     
  Branches       201       417    +216     
===========================================
+ Hits          1020      2131   +1111     

Flag	Coverage Δ
unittests	99.90% <99.87%> (?)

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

Impacted Files	Coverage Δ
datamodel_code_generator/model/__init__.py	100.00% <ø> (ø)
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/base.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/enum.py	100.00% <100.00%> (ø)
...atamodel_code_generator/model/pydantic/__init__.py	100.00% <100.00%> (ø)
...amodel_code_generator/model/pydantic/base_model.py	100.00% <100.00%> (ø)
... and 19 more

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