In contrast to wasm modules, asm.js modules have an empty source URL.
Thus loosen a DCHECK and handle the nullptr source_url correctly.
Also add regression tests that check that we don't crash. Those can
later be extended to check that the profile looks as expected; for now
they only check that we terminate.

R=bmeurer@chromium.org

Bug: chromium:1185919
Change-Id: I6b879f540a2c3647920ad2970efcf7c94712d8c7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2745895Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73313}