Description of problem:

A PipelineRun which has no tasks can is validated without any warnings or errors. This Pipeline is effectively a no-op. 

Prerequisites (if any, like setup, operators/versions):

Steps to Reproduce

Create the attached pipelinerun

Actual results:

The pipeline run is created and completed with 0 tasks

Expected results:

The pipelinerun fails validation with an error like "no tasks"

Reproducibility (Always/Intermittent/Only Once): always

Acceptance criteria: 

Whatever validation is changed should be consistent wherever appropriate. If there are other places where a list of Tasks in a Pipeline spec seems like a non-optional requirement, they should enforce the requirement consistently.

Definition of Done:

Build Details:

Additional info (Such as Logs, Screenshots, etc):

Upstream issue: https://github.com/tektoncd/pipeline/issues/8905

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