patch 8.2.4634: Vim9: cannot initialize a variable to null_list

Problem:    Vim9: cannot initialize a variable to null_list.
Solution:   Give negative count to NEWLIST. (closes #10027)
            Also fix inconsistencies in comparing with null values.
diff --git a/src/vim9compile.c b/src/vim9compile.c
index 1eb6ce2..4e3a1bf 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -1955,7 +1955,7 @@
 		generate_PUSHS(cctx, &li->li_tv.vval.v_string);
 		li->li_tv.vval.v_string = NULL;
 	    }
-	    generate_NEWLIST(cctx, l->lv_len);
+	    generate_NEWLIST(cctx, l->lv_len, FALSE);
 	}
 	list_free(l);
 	p += STRLEN(p);
@@ -2239,10 +2239,10 @@
 			generate_PUSHFUNC(cctx, NULL, &t_func_void);
 			break;
 		    case VAR_LIST:
-			generate_NEWLIST(cctx, 0);
+			generate_NEWLIST(cctx, 0, FALSE);
 			break;
 		    case VAR_DICT:
-			generate_NEWDICT(cctx, 0);
+			generate_NEWDICT(cctx, 0, FALSE);
 			break;
 		    case VAR_JOB:
 			generate_PUSHJOB(cctx);