nillable element support added.

[dapper91]

Adds nillable elements support. See https://www.w3.org/TR/xmlschema-1/#xsi_nil.

Code example:

>>> from typing import Optional
>>> from pydantic_xml import BaseXmlModel, element
>>> 
>>> class Company(BaseXmlModel):
...     title: Optional[str] = element(default=None, nillable=True)
... 
>>> company = Company()
>>> print(company.to_xml(pretty_print=True).decode())
<Company>
  <title xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:nil="true"></title>
</Company>

>>> company = Company(title='SpaceX')
>>> print(company.to_xml(pretty_print=True).decode())
<Company>
  <title>SpaceX</title>
</Company>

Fixes #145 issue.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7f26d33) 91.79% compared to head (af59b91) 91.98%.

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Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #146      +/-   ##
==========================================
+ Coverage   91.79%   91.98%   +0.18%     
==========================================
  Files          25       26       +1     
  Lines        1378     1410      +32     
==========================================
+ Hits         1265     1297      +32     
  Misses        113      113              

Flag	Coverage Δ
unittests	91.98% <100.00%> (+0.18%)	⬆️

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