This prevents the `%builtinweb` or `%builtinfw` skip defaults from
overriding a user's explicitly empty `--web-skip=` or `--fw-skip=`
setting.
This is technically a backwards-incompatible change: Any config that
explicitly sets `--web-skip` or `--fw-skip` to the empty string but
depends on the built-in skip behavior will fail. This is unlikely to
affect many (if any) users; compatibility concerns are believed to be
far less significant than the potential need to turn off the built-in
skip.
