patch 9.0.0457: substitute prompt does not highlight an empty match

Problem:    Substitute prompt does not highlight an empty match.
Solution:   Highlight at least one character.
diff --git a/src/testdir/test_substitute.vim b/src/testdir/test_substitute.vim
index df01e06..92e86a9 100644
--- a/src/testdir/test_substitute.vim
+++ b/src/testdir/test_substitute.vim
@@ -2,6 +2,7 @@
 
 source shared.vim
 source check.vim
+source screendump.vim
 
 func Test_multiline_subst()
   enew!
@@ -684,6 +685,21 @@
   bw!
 endfunc
 
+func Test_sub_highlight_zero_match()
+  CheckRunVimInTerminal
+
+  let lines =<< trim END
+    call setline(1, ['one', 'two', 'three'])
+  END
+  call writefile(lines, 'XscriptSubHighlight', 'D')
+  let buf = RunVimInTerminal('-S XscriptSubHighlight', #{rows: 8, cols: 60})
+  call term_sendkeys(buf, ":%s/^/   /c\<CR>")
+  call VerifyScreenDump(buf, 'Test_sub_highlight_zer_match_1', {})
+
+  call term_sendkeys(buf, "\<Esc>")
+  call StopVimInTerminal(buf)
+endfunc
+
 func Test_nocatch_sub_failure_handling()
   " normal error results in all replacements 
   func Foo()