Add validation for cloudevent uri in Sink

[khrm]

Send cloud event only when CloudEventURI is defined.

Changes

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Release Notes

NONE

[tekton-robot]

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

File	Old Coverage	New Coverage	Delta
pkg/sink/sink.go	73.8%	73.1%	-0.7

[tekton-robot]

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

File	Old Coverage	New Coverage	Delta
pkg/sink/sink.go	73.8%	73.1%	-0.7

[savitaashture]

/lgtm

👍 🎉

[tekton-robot]

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