[SLS-2141] Fix keyError issue when parsing event source

[hghotra]

What does this PR do?

Currently, we use the [] syntax to access keys within the event payload. Switching this to .get(key) will make sure there is no keyError when certain attributes are missing.

Motivation

https://datadoghq.atlassian.net/browse/SLES-764

Testing Guidelines

• Unit tests
• Integration tests

Additional Notes

Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog
• This PR passes the integration tests (ask a Datadog member to run the tests)

[agocs]

I wish there was a linter setting for this kind of thing