regcomp.c: Create NOTHING node when would have been 0 length EXACT

regcomp.c: Create NOTHING node when would have been 0 length EXACT

It's peculiar circumstances indeed that would get to this point in the
code with an EXACT node to be created, but nothing to populate it with.
Perhaps it is impossible; I'm not sure.  But commit
5f820f894e71b6970a5aa0fd763a84b647fd628a changed the behavior, which I
discovered in later re-reading the code.  Probably the node would be
populated with a single NUL.  Just in case it is possible to get here
under these peculiar circumstances, this commit adds code to handle the
case, with a NOTHING node instead of a 0 length EXACT.