maint.mk: fix syntax checks without exclusions

maint.mk: fix syntax checks without exclusions

Commit 727075d0 skipped per-line exclusions if ${exclude} is left
undefined, but was immediately broken by commit 44de969c which
always initialized the variable to the empty string.

* top/maint.mk (_sc_search_regexp): Allow for empty variable.
Reported by Daniel P. Berrange.

Signed-off-by: Eric Blake <eblake@redhat.com>
(cherry picked from commit 3d0ba3fe7865e7030b4d2097f122e64b16f0a5c9)