fix: dont error on nullish prop values in jsx runtime

[duanwilliam]

resolves #10575

Changes

• jsx runtime no longer errors on non-.astro jsx function component instances represented as vnode objects with nullish prop values
  • only try property access on non-nullish prop values when checking if a prop is a slot

Testing

tested locally (ran dev server for @examples/with-mdx and added reproduction sample from linked issue)

not sure where a test file would best go or how to construct it, since particular reproduction from linked issue uses mdx, but don't think mdx is actually necessary, merely an environment where the issue could naturally occur.

Docs

not needed, internal bugfix

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[lilnasy]

Looks great to me! Excellent work finding the fix, I was pleasantly surprised by the investigation in the original report.