Oppiabot should not check further in case that PR is already closed

[Naman-1234]

Please refer to oppia/oppia#12346

Current Behaviour:
In this since that PR was done using develop branch, Oppiabot closed that Pr,but even after closing it checked Linting checks and tagged undefined to add a label.

Expected Behaviour:
Should not check lint checks and assign reviewers for changelabel, If Pr is already closed.

@vojtechjelinek and @jameesjohn Sir Please tell me if this is a valid issue or not. If yes Please do assign me. I would like to work on this one. Thanks!.

[seanlip]

This is a valid issue that can be worked on.

[shubhamsolo]

@seanlip can i work on this issue ? if yes assign this to me

[seanlip]

@shubhamsolo Per the guidance at https://github.com/oppia/oppia/wiki/Contributing-code-to-Oppia#choosing-a-good-first-issue, please provide an explanation of what your PR will do (with names of files you're changing, what you plan to change in each file, etc.), as well as a video showing that the changes work correctly on your local machine. If it looks good, we can assign you to this issue.

See this thread for an example of how another contributor did this: oppia/stale-review-request-notifier#4

Please also follow the other instructions on that wiki page if you have not yet done so. Thanks!