fix: use self-closing tags when children are implictly hidden

[JoshBashed]

Closes #298

Important

Although the issue was generated using AI tools, the PR's code was written by a human.

When a choice case uses _0 = "" to inline an associated value whose contents are XML attributes only, the encoder was emitting an empty open/close pair (<Tag Attr="..."></Tag>) instead of a self-closing tag (<Tag Attr="..." />).

The attributes correctly bubbled up to the parent via formatXMLAttributes, but the empty-keyed phantom child stayed in the elements list, so !elements.isEmpty forced the encoder onto the open/close path.

Changes

• Add isPracticallyEmpty to recursively check if a tag can be self-closing.

Important

formatXMLAttributes doesn't recognize tags with no elements inside children and this PR doesn't change that behavior.

• Use isPracticallyEmpty when deciding whether to self-close an element.
• Add a test case in InlinePropertyTests to ensure the behavior remains.
• Fix existing test cases.