mattermod: panic with nil Issue field

Description

We move the getPRFromComment to an upper function in this commit: https://github.com/mattermost/mattermost-mattermod/commit/9cc7d319e10a1f80d604041ec7425d043fe6bbe2

It is crashing because sometimes `*comment.Issue` is a nil field.

handleAutoAssign never really called PullRequests.Get. It called s.GithubClient.Repositories.ListTeams, so a nil `Issue` field didn't bother it.

We need to properly parse the GitHub response and decide which handler to call depending on it.

/cc

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Mana

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Ibrahim Acikgoz

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Reporter

Agniva De Sarker

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