res_pjsip: Don't require a password when doing userpass authentication.

An empty password is valid for username/password authentication so we should
allow password to be empty/not supplied.

Review: https://reviewboard.asterisk.org/r/3988
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Merged revisions 423481 from http://svn.asterisk.org/svn/asterisk/branches/12
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Merged revisions 423482 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@423483 65c4cc65-6c06-0410-ace0-fbb531ad65f3