set_nametag_attributes: don't reset nametags if text param is not touched

fixes #12901

src/script/lua_api/l_object.cpp

@@ -721,8 +721,7 @@ int ObjectRef::l_set_nametag_attributes(lua_State *L)
 	}
 	lua_pop(L, 1);
 
-	std::string nametag = getstringfield_default(L, 2, "text", "");
-	prop->nametag = nametag;
+	prop->nametag = getstringfield_default(L, 2, "text", prop->nametag);
 
 	prop->validate();
 	sao->notifyObjectPropertiesModified();