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/evalfunc.c b/src/evalfunc.c
index f83559d..5383d09 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -739,6 +739,8 @@
 			ret_number_bool,    f_assert_inrange},
     {"assert_match",	2, 3, FEARG_2,	    NULL,
 			ret_number_bool,    f_assert_match},
+    {"assert_nobeep",	1, 2, FEARG_1,	    NULL,
+			ret_number_bool,    f_assert_nobeep},
     {"assert_notequal",	2, 3, FEARG_2,	    NULL,
 			ret_number_bool,    f_assert_notequal},
     {"assert_notmatch",	2, 3, FEARG_2,	    NULL,