Setting "Homepage and new windows" ignores "Blank Page" value

Steps to reproduce:

1. Run TB 13.0.
2. Open: about:preferences#home
3. Set "Homepage and new windows" to the "Blank Page" value.
4. Restart TB.

Environment: Linux (x86_64).

Expected result: a new TB instance shows about:blank page.

Actual result: a new TB instance shows about:tor page.