Use proper NULL value when we wanna use with XSD

[radeno]

When XML is validated against XSD with nullable=true definition then PHP NULL should be converted to proper XML value.

<element></element> or <element/> is parsed always as empty string. When field is missing then it is not defined. So when value is null, we should provide proper information to XML parsers.

Reference: https://www.w3.org/TR/xmlschema-0/#Nils

Output validated against: https://jsonformatter.org/xml-viewer/a0b80a
XML output converted to XSD schema: https://www.liquid-technologies.com/online-xml-to-xsd-converter

There is a chance that simpler XML parsers could consider xsi:nil="true" still as string

[freekmurze]

Thanks!

[SanderMuller]

@radeno & @freekmurze
This is a breaking change for my application, the API we talk to starting giving errors after this version bump. I tried to see how I could update my code to make it work again, but don't immediately see what change to make to either get non null values or to get the API we talk to to be happy.

Error response from the API:
If the 'nillable' attribute is false in the schema, the 'xsi:nil' attribute must not be present in the instance.

For now I locked this package on the previous release as a temp workaround.

[freekmurze]

@radeno Can you take a look at this problem?

[tw99]

This is a breaking change in our codebase as well. We validate our generated XML against an XSD spec using DOMDocument::schemaValidateSource()

<xsd:element name="address2" type="simpleType.Generic.String.256" minOccurs="0" maxOccurs="1" />

An element with a null value (ex: <address2></address2>) now fails validation with The element is not 'nillable'.

[SanderMuller]

@freekmurze maybe we should revert this for now?

[radeno]

@freekmurze i created PR #226
but i am not sure if it works correctly, because I can't start tests. some PHP error not related to code. Can you try on your side please?

[radeno]

@SanderMuller i created fix PR, hope it will be merged soon.