Fix generation of nullable array items

[tfausten]

This fixes an issue where array items are not generated as Optional, despite adding the nullable: true property: #1496
I'm not sure if this is the best way to fix it, but it works well for me.

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (e9b6edf) 100.00% compared to head (a91fc2f) 99.92%.
Report is 754 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##            master    #1648      +/-   ##
===========================================
- Coverage   100.00%   99.92%   -0.08%     
===========================================
  Files           11       34      +23     
  Lines         1020     3772    +2752     
  Branches       201      889     +688     
===========================================
+ Hits          1020     3769    +2749     
- Misses           0        1       +1     
- Partials         0        2       +2     

Flag	Coverage Δ
unittests	99.57% <99.51%> (?)

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Files	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/arguments.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)
... and 24 more

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