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.