fix(trigger): Make --filter flag truly optional

[dsimansk]

Description

Fix issue with kn trigger create that always required --filter flag.

Changes

• TriggerBuilder adds only empty Spec.Filter for empty or nil arrays
  • The above should also allow builder to be used to clean up existing filters
• kn trigger describe doesn't print empty Filter

Reference

Fixes #744

[knative-prow-robot]

@dsimansk: 0 warnings.

Details

In response to this:

Description

Fix issue with kn trigger create that always required --filter flag.

Changes

• TriggerBuilder adds only empty Spec.Filter for empty or nil arrays
• The above should also allow builder to be used to clean up existing filters
• kn trigger describe doesn't print empty Filter

Reference

Fixes #744

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[dsimansk]

Does it make sense to omit filter in trigger describe command or rather add e.g. EMPTY or N/A?

Current change in this PR is omitting the filter:

➜  client git:(issue-744) ✗ kn trigger describe no-filter             
Name:         no-filter
Namespace:    default
Labels:       eventing.knative.dev/broker=default
Annotations:  eventing.knative.dev/creator=minikube-user, eventing.knative.dev/lastModifier=minik ...
Age:          28m
Broker:       default

Sink:         
  Name:       svc1
  Namespace:  
  Resource:   Service (serving.knative.dev/v1)

Conditions:  
  OK TYPE    AGE REASON

[knative-metrics-robot]

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

File	Old Coverage	New Coverage	Delta
pkg/eventing/v1alpha1/client.go	83.1%	83.9%	0.8
pkg/kn/commands/trigger/describe.go	83.3%	83.7%	0.3

[dsimansk]

/test pull-knative-client-integration-tests

 ++ ./kn service create svc1 --no-wait --image gcr.io/knative-samples/helloworld-go -e TARGET=Knative -n kne2esmoketests
conversion webhook for &{map[apiVersion:serving.knative.dev/v1 kind:Service metadata:map[creationTimestamp:2020-03-18T11:23:20Z generation:1 name:svc1 namespace:kne2esmoketests uid:c14c3d36-c0ad-4dc9-a0dd-16a64a3d097c] spec:map[template:map[metadata:map[annotations:map[client.knative.dev/user-image:gcr.io/knative-samples/helloworld-go] creationTimestamp:<nil> name:svc1-qkwft-1] spec:map[containers:[map[env:[map[name:TARGET value:Knative]] image:gcr.io/knative-samples/helloworld-go name: resources:map[]]]]]]]} failed: Post https://webhook.knative-serving.svc:443/?timeout=30s: no endpoints available for service "webhook"
++ fail_test 

[rhuss]

thanks!

/lgtm

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dsimansk, rhuss

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