Union collection deserialization bugfix

[dapper91]

Union collection deserialization bug fixed.

Deserialization process gets stuck for List[Union] typed field because element index is not incremented if a validation error is raised.

Code example:

from typing import List, Union
from pydantic_xml import BaseXmlModel

class TestSubModel1(BaseXmlModel, tag='submodel1'):
    data: int

class TestSubModel2(BaseXmlModel, tag='submodel2'):
    data: float

class TestModel(BaseXmlModel, tag='model'):
    submodel: List[Union[TestSubModel1, TestSubModel2]]

xml = '''
    <model>
        <submodel2>a</submodel2>
        <submodel1>b</submodel1>
    </model>
'''

TestModel.from_xml(xml)

Fixes the issue.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (ee40d03) 91.78% compared to head (9fce737) 92.00%.

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Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #165      +/-   ##
==========================================
+ Coverage   91.78%   92.00%   +0.22%     
==========================================
  Files          26       26              
  Lines        1509     1514       +5     
==========================================
+ Hits         1385     1393       +8     
+ Misses        124      121       -3     

Flag	Coverage Δ
unittests	92.00% <100.00%> (+0.22%)	⬆️

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