fix: ensure #if blocks correctly guard against nullable prop values

[rChaoz]

Closes #16019, closes #16127

See #16019 for details.

On the failing test (runtime-legacy/binding-this-legacy-component-api): this one fails on main as well when using spread props, it fails on this PR because the logic that previously applied only on spread props logic now always applies. The $$events prop needs to exist somewhere even when using the spread props proxy.

Also, should investigate why #16019 (comment) isn't fixed

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