search: Exclude hidden results from keynav

Since commit 3fb02843, we no longer skip allocation for results that don't fit the width, and give them a 0x0 allocation instead.

That has the unintended side effect of those children now being available to keynav. There are cases where we want 0-sized actors to be part of the focus chain (e.g. FocusTrap), but this isn't one of them, so explicitly exclude 0-sized children from keynav.

Fixes #4470 (closed)