fix(backend): cover nil status race condition from Tekton

[Tomcli]

Which issue is resolved by this Pull Request:
Resolves #

Description of your changes:
Occasionally Tekton may store the pipelineRun to etcd before populating the status field, which it will create a nil pointer panic to the persistent agent. Thus, adding a defer fallback to catch this edge case.

Environment tested:

• Python Version (use python --version):
• Tekton Version (use tkn version): 1.14
• Kubernetes Version (use kubectl version): 1.16
• OS (e.g. from /etc/os-release):

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[kubeflow-bot]

This change is 

[Tomcli]

@adrian555 I just updated the persistent agent image with the this fix, let me know if you still see the nil pointer error.

[animeshsingh]

/assign @adrian555

[animeshsingh]

/assign @drewbutlerbb4

[drewbutlerbb4]

/lgtm
Haven't been able to reproduce past issue in the testing of 5 pipelines across 250+ runs. The persistent agent never crashed throughout the 250+ runs.

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[Tomcli]

@drewbutlerbb4 I fixed the conflicts with the upstream code. Does this look good to you?

[drewbutlerbb4]

Yes, thanks @Tomcli
/lgtm