For CheckNonEmptyString we not only need to rule out that the input is
not the empty string, but also make sure that the input is actually a
string, hence we need to do a proper instance type check in the general
case.

Bug: chromium:949996, chromium:947949, v8:8834, v8:8931, v8:8939, v8:8951
Change-Id: Icc260d735d19337bba4bb71570a6c6385e47c310
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1557146
Auto-Submit: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60681}