Fix (CONCATENATE 'null ...) for generic sequences

Fix (CONCATENATE 'null ...) for generic sequences

 * (CONCATENATE 'NULL SEQUENCE1 SEQUENCE2 ...) ensures that SEQUENCE1,
   SEQUENCE2, ... are empty, but only did so for lists and
   vectors. Instead, use new function EMPTYP which works for all
   sequences. EMPTYP is not exported.

 * Add generic function SEQUENCE:EMPTYP to which EMPTYP dispatches for
   generic sequences. Methods for lists, vectors and generic sequences
   use NULL or (ZEROP (LENGTH ...)).

 * Test cases in seq.impure.lisp.

 * Patch by Jan Moringen; fixes lp#1162301.