Fix nil pointer bug in TaskRun SetDefaults

[mathspanda]

Fix bug issue #351.

If apply a TaskRun CR without TaskRef field, webhook will panic. TaskRef.Kind will be set default value in method SetDefaults, while TaskRef is nil.

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Hi @mathspanda. Thanks for your PR.

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[vdemeester]

/ok-to-test
/lgtm

[mathspanda]

/assign @imjasonh

[nader-ziada]

/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mathspanda, pivotal-nader-ziada, vdemeester

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