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.

src/ex_cmds.c

@@ -4308,6 +4308,10 @@ ex_substitute(exarg_T *eap)
 						  - regmatch.startpos[0].lnum;
 			    search_match_endcol = regmatch.endpos[0].col
 								 + len_change;
+			    if (search_match_lines == 0
+						   && search_match_endcol == 0)
+				// highlight at least one character for /^/
+				search_match_endcol = 1;
 			    highlight_match = TRUE;
 
 			    update_topline();

src/testdir/dumps/Test_sub_highlight_zer_match_1.dump

@@ -0,0 +1,8 @@
+|o+1&#ffffff0|n+0&&|e| @56
+|t|w|o| @56
+|t|h|r|e@1| @54
+|~+0#4040ff13&| @58
+|~| @58
+|~| @58
+|~| @58
+|r+0#00e0003&|e|p|l|a|c|e| |w|i|t|h| @1|.@2|/|a|/|q|/|l|/|^|E|/|^|Y|)|?> +0#0000000&@10|1|,|1| @10|A|l@1| 

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 @@ func Test_sub_cmd_9()
   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()

src/version.c

@@ -703,6 +703,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    457,
 /**/
     456,
 /**/