handle: Process empty LIBGUESTFS_* environment variables correctly (RHBZ#1153515).

Ignore an environment variable:

 - if an environment variable is an empty string, AND

 - if an empty string would have no meaning for that environment
   variable (eg. LIBGUESTFS_BACKEND can only take on certain values,
   and an empty string is not one of them).

Note that LIBGUESTFS_BACKEND_SETTINGS is meaningful as an empty
string, and for this case I have removed the STRNEQ (str, "") test.

Previously:

$ LIBGUESTFS_BACKEND="" guestfish -a /dev/null run
libguestfs: error: invalid backend:

After this commit:

$ LIBGUESTFS_BACKEND="" guestfish -a /dev/null run

(no error message is printed)