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/runtime/doc/testing.txt b/runtime/doc/testing.txt
index 4e4cff0..5d85358 100644
--- a/runtime/doc/testing.txt
+++ b/runtime/doc/testing.txt
@@ -243,7 +243,8 @@
 assert_beeps({cmd})					*assert_beeps()*
 		Run {cmd} and add an error message to |v:errors| if it does
 		NOT produce a beep or visual bell.
-		Also see |assert_fails()| and |assert-return|.
+		Also see |assert_fails()|, |assert_nobeep()| and
+		|assert-return|.
 
 		Can also be used as a |method|: >
 			GetCmd()->assert_beeps()
@@ -377,6 +378,14 @@
 		Can also be used as a |method|: >
 			getFile()->assert_match('foo.*')
 <
+assert_nobeep({cmd})					*assert_nobeep()*
+		Run {cmd} and add an error message to |v:errors| if it
+		produces a beep or visual bell.
+		Also see |assert_beeps()|.
+
+		Can also be used as a |method|: >
+			GetCmd()->assert_nobeep()
+<
 							*assert_notequal()*
 assert_notequal({expected}, {actual} [, {msg}])
 		The opposite of `assert_equal()`: add an error message to