Properly display about:newtab when session is null

[bsclifton]

• 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).

Properly display about:newtab when session is null

Fixes #5379

Auditors: @cezaraugusto

cc: @bbondy, since this does fix the webdriver tests for about:newpage :D

Automated Test Plan (if tests are available for the platform):

1. Open two consoles, both open to the source
2. In first one, run npm run watch-test
3. In other, run `npm run test -- --grep="about:newtab"
4. Watch the tests pass 😄

Manual Test Plan (Windows)

1. Move your session folder out of the way (%appdata%\brave)
2. Launch Brave
3. It should start with about:newtab. Confirm page loads and is showing a picture

[cezaraugusto]

++!!