vim-patch:9.0.0457: substitute prompt does not highlight an empty match (#20186)

Problem:    Substitute prompt does not highlight an empty match.
Solution:   Highlight at least one character.
a04f457a6c

src/nvim/ex_cmds.c

@@ -3887,6 +3887,10 @@ static int do_sub(exarg_T *eap, proftime_T timeout, long cmdpreview_ns, handle_T
                                    - 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(curwin);