Render empty XML nodes for typed nil properties

[OskarEichler]

Summary

With render_nil: true, typed serialization returns nil, which the XML writer passes to Nokogiri or tries to rename. Use the allocated empty node, matching scalar behavior.

Reproduction

require 'representable/xml'
require 'representable/xml/binding'
child=Module.new { include Representable::XML; property :name }
parent=Module.new { include Representable::XML; self.representation_wrap=:parent; property :child, extend: child, render_nil: true }
p Struct.new(:child).new(nil).extend(parent).to_xml
# ArgumentError before; <parent><child/></parent> after.

Verification

• Existing master suite on this isolated branch: 545 tests / 701 assertions, zero failures/errors, three existing skips on Ruby 3.2.11. Ruby 4.0.6 retains the same eight legacy inspection-format failures, zero errors and three skips. Existing tests/expectations are unchanged.
• 80 focused checks on Ruby 3.2.11/4.0.6 for this change. XML first-render uses a separate fresh-process check.
• Combined release-based verification with other focused fixes and attributed Use Setter in OverwriteOnNil if a :setter is defined #177/Hash value of null breaks parsing #264/Update MultiJSON usage to address deprecations #273 source backports: 544 tests / 700 assertions pass on Ruby 3.2.11, plus 4,410 focused checks and 36 boundary checks. Package retains 104 paths and unchanged compared metadata. All 36 runtime files compile on both Rubies; no newly introduced default Lint offenses.

Breaking changes / limitations

Typed nil values explicitly requesting render_nil produce empty tags. Default omission is unchanged. No new semantics are defined for an entirely nil collection.

No new/modified checked-in tests. Ruby 2.4–3.1, JRuby, Windows and Linux were not executed locally. Ruby >=2.4 and runtime dependency ranges remain unchanged. External test tooling supplies a legacy MiniTest alias and ruby-prof 1.7.x; no assertions were changed. No production verification is claimed.