Add new `target` fallback for INP event

See: https://chromestatus.com/feature/5143499213242368 (not shipped yet).

We should continue to use our own selector logic where possible as it's more complete (and customizable), but fallback to this one when the node is empty so that can't be used.

Start by reviewing the existing INP event target selector logic and the linked ChromeStatus feature 5143499213242368. Preserve the current selector where it returns a node, and use the new target only when the node is empty; verify the resulting target behavior with the relevant existing tests.

Written by the indexing model from the issue text.