xsd:string text nodes should have empty string as default value

[tefra]

Early on I made the decision to treat empty values as None for text nodes, which is not correct, xml parsers obviously don't know about the schema but xsdata can make the distinction between empty content and nil content.

• Make text nodes required unless are marked explicitly nillable
• Separate bindings for nillable types and nillable elements (nillable elements are None, nillable types are objects with None text values)
• Cascade xs:element default/fixed/nillable properties only to text nodes
• Explicitly mark as optional nillable nodes even if they have a default value
• Set default value to empty string for str text nodes if not set (wip)
• ElementMapper should handle xsi:nil attributes as well

todos

• I am still debating if this should be a feature that can be enabled/disabled. I am thinking default On / Strict
• I still have some failures in w3c test suite regarding text nodes derived from xs:any, xs:anySimpleType, ... and kw_only dataclasses

Resolves hopefully #567

[codecov]

Codecov Report

Merging #570 (14b96a6) into master (56e885e) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #570   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           93        93           
  Lines         7965      7983   +18     
  Branches      1368      1373    +5     
=========================================
+ Hits          7965      7983   +18     

Impacted Files	Coverage Δ
xsdata/utils/testing.py	100.00% <ø> (ø)
xsdata/codegen/container.py	100.00% <100.00%> (ø)
xsdata/codegen/handlers/__init__.py	100.00% <100.00%> (ø)
xsdata/codegen/handlers/attribute_default_value.py	100.00% <100.00%> (ø)
xsdata/codegen/handlers/attribute_normalize.py	100.00% <100.00%> (ø)
xsdata/codegen/handlers/class_extension.py	100.00% <100.00%> (ø)
xsdata/codegen/mappers/element.py	100.00% <100.00%> (ø)
xsdata/codegen/models.py	100.00% <100.00%> (ø)
xsdata/formats/dataclass/filters.py	100.00% <100.00%> (ø)
xsdata/formats/dataclass/parsers/bases.py	100.00% <100.00%> (ø)
... and 2 more

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