optional raw element bug fixed.

[dapper91]

Fixes the bug with optional raw element.

Example:

>>> class Model(BaseXmlModel, tag='model', arbitrary_types_allowed=True):
...     element1: Optional[ElementT] = element(default=None)
...
>>> Model().to_xml()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.11/site-packages/pydantic_xml/model.py", line 399, in to_xml
    return etree.tostring(self.to_xml_tree(skip_empty=skip_empty), **kwargs)
                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/pydantic_xml/model.py", line 380, in to_xml_tree
    self.__xml_serializer__.serialize(
  File "/usr/local/lib/python3.11/site-packages/pydantic_xml/serializers/factories/model.py", line 170, in serialize
    field_serializer.serialize(
  File "/usr/local/lib/python3.11/site-packages/pydantic_xml/serializers/factories/raw.py", line 38, in serialize
    sub_element = element.from_native(value)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/pydantic_xml/element/native/lxml.py", line 22, in from_native
    tag=element.tag,
        ^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'tag'

Expected behavior:

>>> from pydantic_xml import BaseXmlModel, element
>>> from xml.etree.ElementTree import Element
>>> from typing import Optional
>>> 
>>> class Model(BaseXmlModel, tag='model', arbitrary_types_allowed=True):
...     element1: Optional[Element] = element(default=None)
... 
>>> print(Model().to_xml().decode())
<model/>

fixes the issue #158

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (73228d7) 91.71% compared to head (c5e77d5) 91.77%.

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Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #160      +/-   ##
==========================================
+ Coverage   91.71%   91.77%   +0.06%     
==========================================
  Files          26       26              
  Lines        1508     1508              
==========================================
+ Hits         1383     1384       +1     
+ Misses        125      124       -1     

Flag	Coverage Δ
unittests	91.77% <100.00%> (+0.06%)	⬆️

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