Welcome Screen: Don't allow unlocking with empty passphrase

Attempting to unlock with an empty passphrase failed with:

Failed to unlock the Persistent Storage. Please start Tails and send an error report.

and made the unlock button non-sensitive, forcing the user to reboot to unlock their Persistent Storage.

We now only make the unlock button sensitive when the passphrase is non-empty.

Closes #19357 (closed)

TODO:

• Check Jenkins

This is based on !1060 (merged), please review and merge after that!