Description of problem:

When referencing a nested attribute from the event body in an on-cel annotation in PaC, fields which have a zero value become invalid/missing keys if referenced. e.g. event.PullRequestLabels is an empty when the pull request has no labels, so body.pull_request.labels causes the CEL expression fail execution.

Workaround
Nested attributes like this have to be accessed with (has(body.pull_request.labels) && body.pull_request.labels...).

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

Steps to Reproduce

• Author a pipelinerun with an on-cel expression which references `body.pull_request.labels`
• Create a pull request with this pipelinerun which does not include any labels

Actual results:

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

Expected results:

The pipelinerun does not start (if the on-cel does not match) but does not have any errors

Reproducibility (Always/Intermittent/Only Once): always

Acceptance criteria: 

• the on-cel expression does not fail to evaluate when a referenced field is present but empty in the event.

Definition of Done:

• acceptance criteria is met
• unit tests are written
• KONFLUX-9908 is updated accordingly

Build Details:

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

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