Check if the application session key is available before access

[adriansmares]

Summary

References https://www.thethingsnetwork.org/forum/t/internal-server-error/44472/3

This quickfix ensures that as.skipPayloadCrypto does not panic if a session is available, but the AppSKey is nil.

Changes

• Add a nil-check for session.AppSKey

Testing

Tested locally with an unset AppSKey.

Regressions

Not expected.

Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• Compatibility: The changes are backwards compatible with existing API, storage, configuration and CLI, according to the compatibility commitments in README.md for the chosen target branch.
• Documentation: Relevant documentation is added or updated.
• Changelog: Significant features, behavior changes, deprecations and fixes are added to CHANGELOG.md.
• Commits: Commit messages follow guidelines in CONTRIBUTING.md, there are no fixup commits left.