Caret-shape: applies to empty element that accepts text input.

As per [1], the caret-shape property only applies to text or elements
that accept text input. This CL handles the case of empty element.

[1] https://drafts.csswg.org/css-ui/#caret-shape

Bug: 353713061
Change-Id: Id21808b5bf6581cb77e2768a613628373accf7ab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6875245
Reviewed-by: Stephen Chenney <schenney@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Ziran Sun <zsun@igalia.com>
Cr-Commit-Position: refs/heads/main@{#1506324}