BUILD: map: fix build warning

<pattern> field pointer of pat_ref_elt structure has been by a
zero-length array. As such, it's now unneeded to check for NULL address
before printing it.

This type conversion was done in the following commit :
  3ac9912837
  OPTIM: pattern: save memory and time using ebst instead of ebis

The current patch is mandatory to fix the following GCC warning :
  CC      src/map.o
  src/map.c: In function ‘cli_io_handler_map_lookup’:
  src/map.c:549:54: error: the comparison will always evaluate as ‘true’ for the address of ‘pattern’ will never be NULL [-Werror=address]
  549 |                                         if (pat->ref && pat->ref->pattern)
      |

No need to backport it unless the above commit is.

src/map.c

@@ -546,13 +546,13 @@ static int cli_io_handler_map_lookup(struct appctx *appctx)
 
 				/* display pattern */
 				if (ctx->display_flags == PAT_REF_MAP) {
-					if (pat->ref && pat->ref->pattern)
+					if (pat->ref)
 						chunk_appendf(&trash, ", key=\"%s\"", pat->ref->pattern);
 					else
 						chunk_appendf(&trash, ", key=unknown");
 				}
 				else {
-					if (pat->ref && pat->ref->pattern)
+					if (pat->ref)
 						chunk_appendf(&trash, ", pattern=\"%s\"", pat->ref->pattern);
 					else
 						chunk_appendf(&trash, ", pattern=unknown");