Fix about:blank appearing on new tab in new windows

[diracdeltas]

fix #15162

openableByContextMenu returned false if URL is empty for paranoia
reasons, but it turns out this breaks some edge cases. See
#15061 for instance.

Test Plan:

1. unit tests pass
2. on MacOS, close all windows
3. click 'New tab'
4. new window should appear and show the new tab page
5. repeat step 3 and 4 with private tab, tor tab, session tab, and new window

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed. (Ask a Brave employee to help if you cannot access this document.)

Test Plan:

Reviewer Checklist:

• Request a security/privacy review as needed if one was not already requested.

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[bsclifton]

When reviewing, I'll check if this fixes #15134 also

[diracdeltas]

pretty sure it won't fix #15134 since that relates to file:/// URLs

[bsclifton]

Works great 😄 👍

[bsclifton]

master 141e002
0.24.x 1f1f1d9
0.23.x c5dd2a9