Treat missing XML attributes as missing fragments

[OskarEichler]

Summary

Attribute reading returns nil for absence, causing overwrite-on-nil to erase an existing value. Return FragmentNotFound as other bindings do. Addresses the missing-attribute path from #200.

Reproduction

require 'representable/xml'
mod=Module.new { include Representable::XML; property :name, attribute: true }
record=Struct.new(:name).new('original').extend(mod)
p record.from_xml('<item/>').name # nil before; 'original' 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.
• 100 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

Absent attributes preserve existing values or apply an explicit default. Present empty-string attributes still set an empty string.

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.