don't set a user if there is no user

Summary:
it doesn't have technical downsides but ends up creating urls of the type

scheme://@host/path

which is technically equal to scheme://host/path, so the excess @ is really
just not necessary. simply check if the username is empty and if so
do not set a username on the QUrl

Test Plan:
- no useless @
- setting up fish connection with and without username still works (without defaults to local username)

Reviewers: broulik

Reviewed By: broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D16275