fix: stale session ids being displayed in the attach to session dropdown

[huzaifaiftikhar]

1. Attach to session placeholder text enterYourSessionId was not getting displayed as we were setting the value to '' in case there are no running sessions, however, the antd expects this value to be undefined instead. For reference check this stackoverflow link.
2. Whenever the GET_SESSIONS_DONE event is received for the first time we set the attachSessId inside the state variable, however, for the subsequent occurrences of this event we never update the state.attachSessId as we had already set that once initially. This leads to stale sessions that are not running anymore and still getting listed. This stale entry is only removed if the user selects a valid entry from the dropdown so that the SET_ATTACH_SESS_ID event is triggered which updates the value of state.attachSessId, however, there could be scenarios where there are no running sessions anymore and therefore in this situation, there's no way to to get rid of this stale entry.
3. To fix the above issue I have created a method isAttachSessIdValid that validates whether the existing value of state.attachSessId is any more valid or not? If it's not valid then we update its value with the currently running session id. If there are no running sessions currently then we reset the value to null (which is the initial state) so that the placeholder text starts getting displayed.