vim-patch:8.2.2694: when 'matchpairs' is empty every character beeps (#14279)

Problem:    When 'matchpairs' is empty every character beeps. (Marco Hinz)
Solution:   Bail out when no character in 'matchpairs' was found.
            (closes vim/vim#8053)  Add assert_nobeep().
5b8cabfef7

src/nvim/search.c

@@ -2326,6 +2326,9 @@ showmatch(
       return;
     }
   }
+  if (*p == NUL) {
+    return;
+  }
 
   if ((lpos = findmatch(NULL, NUL)) == NULL) {  // no match, so beep
     vim_beep(BO_MATCH);