patch 8.2.2694: when 'matchpairs' is empty every character beeps

Problem:    When 'matchpairs' is empty every character beeps. (Marco Hinz)
Solution:   Bail out when no character in 'matchpairs' was found.
            (closes #8053)  Add assert_nobeep().
diff --git a/src/search.c b/src/search.c
index d8c21f4..37ccc37 100644
--- a/src/search.c
+++ b/src/search.c
@@ -2817,6 +2817,8 @@
 	if (*p == NUL)
 	    return;
     }
+    if (*p == NUL)
+	return;
 
     if ((lpos = findmatch(NULL, NUL)) == NULL)	    // no match, so beep
 	vim_beep(BO_MATCH);