fix: avoid NaN keyframe values in slide transition

[Nic-Polumeyv]

Fixes #14205

When a slide transition runs on an element without a layout box, for example inside a display: none parent, getComputedStyle returns values like auto for the animated dimensions. These were passed through parseFloat unguarded, so the generated css contained height: NaNpx and the browser rejected the keyframe property with an "Invalid keyframe value" warning on every transition.

Svelte 4 had the same NaN but never surfaced it, because invalid declarations in injected stylesheet rules are silently dropped, while the Web Animations API validates keyframes and warns.

This treats unparseable computed dimensions as 0. The user visible result is the same as today (the browser was already discarding the invalid property, and an element with no layout box has nothing to slide), minus the console spam.

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