fix: keep nil values during (de)serialization

[mirkoSekulic]

Description

Sets IsNullable = true to in XmlElement attribute for reference properties if they are defined as nillable in xsd.

Known limitations

Since we're using c# model, when desiralizing XML and serializing it back back, we can't always reproduce the same same XML.

Example

According to xsd schema:

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<xsd:schema attributeFormDefault="unqualified"
            elementFormDefault="qualified"
            xmlns:xsd="http://www.w3.org/2001/XMLSchema"
            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <xsd:element name="root" type="SampleType" />

  <xsd:complexType name="SampleType">
    <xsd:all>
      <xsd:element name="refElement" type="xsd:string" nillable="true" minOccurs="0" maxOccurs="1"/>
    </xsd:all>
  </xsd:complexType>
</xsd:schema>

following XML is valid since minOccurs is set to "0":

<?xml version="1.0" encoding="utf-8"?>
<root xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
</root>

But when deserializing and serializing it back we'll produce following XML:

<?xml version="1.0" encoding="utf-8"?>
<root xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <refElement xsi:nil="true" />
</root>

Since the node type is string, functionally XMLs will be equivalent.

Related Issue(s)

Altinn/app-lib-dotnet#911

• "nil=true" in XML disapears for elements of type xs:date app-lib-dotnet#911

Verification

• Your code builds clean without any errors or warnings
• Manual testing done (required)
• Relevant automated test added (if you find this hard, leave it and we'll help out)

Documentation

• User documentation is updated with a separate linked PR in altinn-studio-docs. (if applicable)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.31%. Comparing base (498673f) to head (91b054f).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #14127   +/-   ##
=======================================
  Coverage   95.31%   95.31%           
=======================================
  Files        1776     1776           
  Lines       23130    23130           
  Branches     2687     2687           
=======================================
  Hits        22046    22046           
  Misses        835      835           
  Partials      249      249           

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