Must have struct for json to work, not nil

[yorinasub17]

Description

Fixes a bug in the new github package to properly handle the json unmarshal. It turns out using a nil uninitialized pointer is not supported (leads to the following runtime error):

Error loading configured Github App Config JSON: json: Unmarshal(nil *github.GithubAppConfig)

TODOs

Please ensure all of these TODOs are completed before asking for a review.

• Ensure the branch is named correctly with the issue number. e.g: feature/new-vpc-endpoints-955 or bug/missing-count-param-434.
• Update the docs.
• Keep the changes backward compatible where possible.
• Run the pre-commit checks successfully.
• Run the relevant tests successfully.
• Ensure any 3rd party code adheres with our license policy or delete this line if its not applicable.

[zackproser]

LGTM

[yorinasub17]

Thanks for review! Merging now.