[Extensions] Fix a crash with isolated world CSP

The isolated world CSP for scripts injected from an extension may be
a specific CSP (as is the case for MV3 extensions), an empty CSP (for
MV2 extensions), or a null CSP, indicating the script should use the
main world CSP (the case for extensions converted from users scripts).
The latter caused a crash, since we were dereferencing an empty
optional, and it turned out we didn't have any (!) browser tests for
script injection by extensions converted from user scripts.

This CL fixes the crash and adds a basic end-to-end test for
user-script-converted extensions injecting into a web page.

Fixed: 377725082
Change-Id: I8f434c4f16c766d0b1bbf20c429d85b3d661f875
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6001219
Commit-Queue: Devlin Cronin <rdevlin.cronin@chromium.org>
Reviewed-by: Tim <tjudkins@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1380027}