Add test confirming PageState base urls aren't empty.

ExplodedFrameState::initiator_base_url_string is absl::optional, when
the feature is off, and should never be empty when it's enabled, so
there's no need to store empty GURLs in session state. This CL
1) adds a comment in RecursivelyGenerateFrameState() to clarify that
   we're not storing an empty GURL, and
2) adds a test to confirm that we don't save PageState with an empty
   GURL in place of a nullopt.

Bug: 1356658
Change-Id: Idf8ee754dd51985f0a25ece6eb480b0ee71b9463
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4455088
Reviewed-by: Charlie Reis <creis@chromium.org>
Commit-Queue: James Maclean <wjmaclean@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1150738}