fix nil pointer

[jkhelil]

Changes

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Release Notes

NONE

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[lcarva]

Changes look good. Can you update the release notes in the PR description to include more information about the issue? This is used to automatically generate release notes when we cut a release.

Nevermind. PR now closed.

[tekton-robot]

The following is the coverage report on the affected files.
Say /test pull-tekton-chains-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/chains/formats/slsa/v1/pipelinerun/pipelinerun.go	75.9%	74.4%	-1.4