fix changed bahaviour of [filter]x="" [/filter]

in 1.12 [filter]x="" [/filter] filter machtes no units while in 1.13. it maches all units, this was likely caused by a6decea, this commit should also fix #1992 where this behaviour change caused an infinite [while] loop in an addons wml code.

src/units/filter.cpp

@@ -55,7 +55,10 @@ struct unit_filter_xy : public unit_filter_base
 		std::string x = utils::interpolate_variables_into_string(x_, *(resources::gamedata));
 		std::string y = utils::interpolate_variables_into_string(y_, *(resources::gamedata));
 
-		if(x == "recall" && y == "recall") {
+		if(x.empty() && y.empty()) {
+			return false;
+		}
+		else if(x == "recall" && y == "recall") {
 			return !args.fc->get_disp_context().map().on_board(args.loc);
 		}
 		else {