[O] Use default value if a variable is not defined

[Nathan Neff]

Hello,

I'm trying to create a function that will tell org-capture
dynamically where to put the captured item.

I have it almost working -- where I'm getting hung up on is
with basic Emacs lisp.  I want to basically implement this
pseudo code:

IF I have defined a variable called njn/current-q-file then
        (find-file njn/current-q-file)
ELSE
       (find-file ("questions.org")))



However, if I do not define my njn/current-q-file then
I get a "Symbol's value as variable is void" error when Emacs starts up.

How do I test for the existence (and non-nil ness of a variable in Emacs)

Thanks,
--Nate