fix: initialize variables to prevent potentially uninitialized usage

Fixed compiler warnings about potentially uninitialized variables in
handle_key_down and hkm_unregister functions by adding else clauses
to set default empty string values.

Author: dongfengweixiao

packages/hotkey_manager_linux/linux/hotkey_manager_linux_plugin.cc

@@ -40,6 +40,8 @@ void handle_key_down(const char* keystring, void* user_data) {
 
   if (result != hotkey_id_map.end())
     identifier = result->first.c_str();
+  else
+    identifier = "";
 
   g_autoptr(FlValue) event_data = fl_value_new_map();
   fl_value_set_string_take(event_data, "identifier",
@@ -111,6 +113,8 @@ static FlMethodResponse* hkm_unregister(_HotkeyManagerLinuxPlugin* self,
 
   if (result != hotkey_id_map.end())
     keystring = result->second.c_str();
+  else
+    keystring = "";
 
   keybinder_unbind(keystring, handle_key_down);
   hotkey_id_map.erase(identifier);