fix: handle invalid values and empty tag names in XMLFormatter

[gr8man]

Description
Fixes a bug in XMLFormatter where an array key that becomes empty after sanitization threw a ValueError, and a value of a non-stringable object caused a fatal Error. Such keys now fall back to the tag name item, and non-stringable values throw a FormatException.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[carson-codeigniter4]

Hi there, @gr8man! 👋

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