Description of problem:

When referencing a body.pull_request in a PipelineRun to match pull_request event and the `/test` or `/retest` comment is made on that pull request, it is causing error "failed to evaluate: no such key: pull_request" because event payload of comment event doesn't contain pull_request field.

Prerequisites (if any, like setup, operators/versions):

Steps to Reproduce

• Author a pipelinerun with an on-cel expression which references `body.pull_request`
• Make a GitOps comment on the pull request e.g. `/test`

Actual results:

The pipelinerun fails to start with an error like CEL expression evaluation error: expression }}-- {{failed to evaluate: no such key: pull_request

Reproducibility (Always/Intermittent/Only Once): always

Acceptance criteria: 

• the on-cel expression does not fail to evaluate when on-cel annotation contains body.pull_request on comment event.

Definition of Done:

• acceptance criteria is met
• unit tests are written

Build Details:

Additional info (Such as Logs, Screenshots, etc):

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