Add nil Check for Completion time in Finalizer

[khrm]

There are cases when TaskRun don't have completion time. e.g. : CreateContainerConfigError

Changes

Submitter Checklist

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• [ ] Has Docs included if any changes are user facing
• [ ] Has Tests included if any functionality added or changed
• [ ] Tested your changes locally (if this is a code change)
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Release Notes

[khrm]

/cherry-pick release-v0.12.x

[tekton-robot]

@khrm: once the present PR merges, I will cherry-pick it on top of release-v0.12.x in a new PR and assign it to you.

In response to [this](https://github.com/tektoncd/results/pull/859#issuecomment-2429440042): >/cherry-pick release-v0.12.x Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes/test-infra](https://github.com/kubernetes/test-infra/issues/new?title=Prow%20issue:) repository.

[khrm]

/assign @vdemeester

[khrm]

/assign @enarha

[khrm]

/assign @avinal

[tekton-robot]

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

File	Old Coverage	New Coverage	Delta
pkg/watcher/reconciler/pipelinerun/reconciler.go	30.5%	29.0%	-1.5

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: avinal

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[tekton-robot]

@khrm: new pull request created: #860

In response to [this](https://github.com/tektoncd/results/pull/859#issuecomment-2429440042): >/cherry-pick release-v0.12.x Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes/test-infra](https://github.com/kubernetes/test-infra/issues/new?title=Prow%20issue:) repository.