#4 Support blank (null) passwords

[Grant McDorman]

Some VNC servers support multiple passwords, e.g. for
full control and view-only sessions. It is possible,
for these servers, to have one of the passwords set
to blank (effectively, no password required). In this
case, the server will ask for a password, and the
client must respond with a blank password. Support
for blank passwords must be added to vncsnapshot.