kn trigger create fails without --filter flag

[dsimansk]

Bug report

There was a bug in Eventing 0.13.0 that rejected triggers without filters. However, even on version 0.13.1 the creation of such trigger will fail. There's a bug in our current impl that will initialize empty trigger.Spec.Filter.Attributes regardless of no flag provided that causes the error behaviour.

Beware, that kn trigger describe has to be fixed for empty filters also.

$ kn trigger create test --broker default --sink svc:svc1
cannot create trigger 'test' in namespace 'default' because: admission webhook "validation.webhook.eventing.knative.dev" denied the request: validation failed: At least one filtered attribute must be specified: spec.filter.attributes

Expected behavior

Trigger can be created without --filter flag.

Steps to reproduce the problem

1. kn trigger create test --broker default --sink svc:svc1

kn version

HEAD

Knative (serving/eventing) version

0.13.1

/kind good-first-issue

[mvinkler]

Also, kn trigger create -h should be updated accordingly, now it suggests --filter is mandatory:

[root@ocp-dynamic-7202 eventing]# kn trigger create -h
Create a trigger

Usage:
  kn trigger create NAME --broker BROKER --filter KEY=VALUE --sink SINK [flags]

[dsimansk]

/assign dsimansk