Stdlib: XMLWriter null string args soft-deprecate like Zend (#31610)

[PurHur]

Summary

• Soft-null E_DEPRECATED + coerce for XMLWriter / xmlwriter_* Z_PARAM_STR args (php-src php_xmlwriter.c), same shape as SimpleXML Regression: SimpleXMLElement::addChild/addAttribute(null) TypeError — Zend Deprecated + ValueError empty (ext/simplexml/sxe.c) #31554.
• Match Zend empty-name ValueError ordinals (Argument #2 quirks for startElement / writeElement / startAttribute / writeAttribute).
• Closes Regression: XMLWriter string args (null) skip soft-null DEP + ValueError arg shape — Zend php_xmlwriter.c #31610

Test plan

• ./script/docker-exec.sh -- bash -lc 'php test/repro/maintainer_gap_xmlwriter_null_soft.php' (Zend baseline)
• ./script/docker-exec.sh -- bash -lc 'php bin/vm.php test/repro/maintainer_gap_xmlwriter_null_soft.php'
• ./script/docker-exec.sh -- bash -lc 'php bin/jit.php test/repro/maintainer_gap_xmlwriter_null_soft.php'
• vendor/bin/phpunit test/compliance/XmlwriterNullStringArgsSoft31610VMTest.php — OK
• vendor/bin/phpunit test/compliance/XmlwriterNullStringArgsSoft31610JITTest.php — OK

Verification transcript (VM = Zend)

DEP:XMLWriter::startElement(): Passing null to parameter #1 ($name) of type string is deprecated
ValueError: XMLWriter::startElement(): Argument #2 must be a valid element name, "" given
… (writeElement/startAttribute/writeAttribute + content methods ok)

php-src-strict; no new runtime/*.c.

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