Hermes: fix adding job types form

Hermes: fix adding job types form

This is a dirty work around to a Horde_Form behavior.
Horde_Variables#exists() only checks on expected variables, while
Horde_Variables#get() returns the value if one was passed.  Since the form
name itself isn't "expected", exists() returns false.  This work around
makes this multi-form page work again.