[dialog] fix dropping empty string dlg vals

Upon DB recovery (after an OpenSIPS restart) do not drop emtpy string dlg vals - as we do allow creation of such vals via cfg and api and have them saved to DB, it makes all the sense to also restore them
This triggers some startup warnings when using in-dialog re-INVITE probing.
Closes #3108

modules/dialog/dlg_db_handler.c

@@ -427,8 +427,6 @@ void read_dialog_vars(char *b, int l, struct dlg_cell *dlg)
 		p = read_pair( p, end, &name, &isval, &type);
 		if (p==NULL) break;
 
-		if (isval.s.len==0) continue;
-
 		LM_DBG("new var found  <%.*s>\n",name.len,name.s);
 
 		/* add the variable */