EventListener spec validation is missing

[savitaashture]

Expected Behavior

• Creating EventListener should fail if provided spec is empty
• Its expected that atleast atleast one of the label selector, namespace selector, or triggers (or triggerGroup) is provided

Actual Behavior

• Creating EventListener is success if provided spec is empty and creates EL pod but that doesn't do any functionality.

Steps to Reproduce the Problem

1. Create EL with empty spec
2. Pod creation success
3. sending request to EL pod fails

Additional Info

• Kubernetes version:

  Output of kubectl version:

kubectl version
Client Version: version.Info{Major:"1", Minor:"20", GitVersion:"v1.20.2", GitCommit:"faecb196815e248d3ecfb03c680a4507229c2a56", GitTreeState:"clean", BuildDate:"2021-01-13T13:28:09Z", GoVersion:"go1.15.5", Compiler:"gc", Platform:"linux/amd64"}
Server Version: version.Info{Major:"1", Minor:"22", GitVersion:"v1.22.3", GitCommit:"c92036820499fedefec0f847e2054d824aea6cd1", GitTreeState:"clean", BuildDate:"2021-10-27T18:35:25Z", GoVersion:"go1.16.9", Compiler:"gc", Platform:"linux/amd64"}  

• Tekton Pipeline version:

  Output of tkn version or kubectl get pods -n tekton-pipelines -l app=tekton-pipelines-controller -o=jsonpath='{.items[0].metadata.labels.version}'