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();