QNetworkAccessManager FTP - authenticationRequired signal should be asking for password if URL contains only a username

RFC1738 states:
If the URL supplies a user name but no password, and the remote
server requests a password, the program interpreting the FTP URL
should request one from the user.

Current behaviour is that login is attempted using "anonymous@" as the password

Correct behaviour:
after receiving 331 response, emit authenticationRequired (prefill the QAuthenticator username with the username that was used)

Maybe easier implementation:
emit authenticationRequired before queuing the login

Test cases:
ftp://ftptest@qt-test-server/... (should prompt for password)
ftp://ftptest:@qt-test-server/... (should attempt login with "" as password?)