LP#1400082 - avoid uninitialized var warning at TPAC login

Give username a default value of '', which makes the regex happy (avoiding an
uninit warning in Apache logs if no value was entered) and still counts as de
nada for the purposes of the actual login dance.

Signed-off-by: Dan Scott <dscott@laurentian.ca>
Signed-off-by: Ben Shum <bshum@biblio.org>