Make def:<var> compatible between "static" and "dynamic" vars.

Some variables are simple pointers to internal memory (e.g.
vtype_stringptr), other variables are dynamically filled with content
(e.g. vtype_string_func).

The static variables contain "" if they're not defined yet.
At least on dynamic variable (recipients, backed by fn_recipients())
returned NULL instead of "", which lead to unexpected results on
def:recipients.

To keep the functions usable in places where it makes sense to return
NULL and "" as distinct values, I didn't touch the functions, but the
evaluation logic in find_variable().