fix(github): Prevent AttributeError from null webhook fields

[sentry]

GitHub webhooks can send payloads where the `installation` or `repository` fields are explicitly `null` instead of being absent. Previously, code using `event.get("key", {}).get("id")` would fail with an `AttributeError` because `event.get("key")` would return `None` (since the key exists) and then `.get("id")` would be called on `None`.

This change modifies the affected code in get_github_external_id and get_scm_stream_extra to use (event.get("key") or {}).get("id"). This ensures that if event.get("key") returns None, it correctly falls back to an empty dictionary, preventing the AttributeError.

Legal Boilerplate

Look, I get it. The entity doing business as "Sentry" was incorporated in the State of Delaware in 2015 as Functional Software, Inc. and is gonna need some rights from me in order to utilize my contributions in this here PR. So here's the deal: I retain all rights, title and interest in and to my contributions, and by keeping this boilerplate intact I confirm that Sentry can use, modify, copy, and redistribute my contributions, under Sentry's choice of terms.

Fixes SENTRY-5RGG

_{Comment @sentry <feedback> on this PR to have Autofix iterate on the changes.}