update entities.spawn(...) semantics to the docs state

src/logic/scripting/lua/libentity.cpp

@@ -20,9 +20,12 @@ static int l_spawn(lua::State* L) {
     auto defname = lua::tostring(L, 1);
     auto& def = content->entities.require(defname);
     auto pos = lua::tovec3(L, 2);
-    dynamic::Value args = dynamic::NONE;
+    dynamic::Map_sptr args = nullptr;
     if (lua::gettop(L) > 2) {
-        args = lua::tovalue(L, 3);
+        auto value = lua::tovalue(L, 3);
+        if (auto map = std::get_if<dynamic::Map_sptr>(&value)) {
+            args = *map;
+        }
     }
     level->entities->spawn(def, pos, args);
     return 1;