Fix startup packet length when username is null (#111)
Fix startup packet length when username is null (#111)

`username` is an optional parameter in the connection `open` method but
the startup packet length was calculated as the `username` was always
not null. This patch fix that behavior, leading to a more clear (backend
raised) error message when you don't pass the username field on network
connections.
stablekernel/postgresql-dart,stablekernel/postgresql-dart