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/testdir/test_textformat.vim b/src/testdir/test_textformat.vim
index 32ff3cc..0dc28c7 100644
--- a/src/testdir/test_textformat.vim
+++ b/src/testdir/test_textformat.vim
@@ -858,6 +858,14 @@
   close!
 endfunc
 
+func Test_empty_matchpairs()
+  split
+  set matchpairs= showmatch
+  call assert_nobeep('call feedkeys("ax\tx\t\<Esc>", "xt")')
+  set matchpairs& noshowmatch
+  bwipe!
+endfunc
+
 func Test_mps_error()
   let encoding_save = &encoding