#12 using defined for conditionals and loop context vars

[Tekkie]

Template conditionals were not using 'defined' to
check variable existance, so variables with a value of
zero resulted in false positives.

Loop context vars were intialized with zero-values
instead of undefined values, causing false positives
when the conditional checking was fixed.

Patch adds defined to the 4 lines setting the jump
address within the HTML::Template::output method, and
changes zero values to undefined in the
HTML::Template::LOOP::output method.

[Tekkie]

fixes zero-value checking in TMPL conditionals