patch 9.0.1092: search error message doesn't show used pattern

Problem:    Search error message doesn't show used pattern.
Solution:   Pass the actually used pattern to where the error message is
            given. (Rob Pilling, closes #11742)
diff --git a/src/testdir/test_global.vim b/src/testdir/test_global.vim
index 1c76642..34857b2 100644
--- a/src/testdir/test_global.vim
+++ b/src/testdir/test_global.vim
@@ -92,6 +92,18 @@
   close!
 endfunc
 
+func Test_global_empty_pattern()
+  " populate history
+  silent g/hello/
+
+  redir @a
+  g//
+  redir END
+
+  call assert_match('Pattern not found: hello', @a)
+  "                                     ^~~~~ this was previously empty
+endfunc
+
 " Test for global command with newline character
 func Test_global_newline()
   new