Fix list_event_for_all_namespaces when event_time is None

[Rishit-dagli]

What type of PR is this?

/kind bug

What this PR does / why we need it:

As discussed in #1826 the following error appears when when using list_event_for_all_namespaces:

from kubernetes import config, client

config.load_kube_config()
api = client.EventsV1beta1Api()
print(api.list_event_for_all_namespaces())

The error:

...
ValueError: Invalid value for `event_time`, must not be `None`

This could easily be validated by targeting a minikube cluster using minikube start, which is also on what I tested the changes this PR makes. The API supports using None for the event_time. This PR removes the check to do so. I tested this out locally for minikube clusters and the tests pass as well.

Which issue(s) this PR fixes:

Fixes #1826

Does this PR introduce a user-facing change?

None

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

[k8s-ci-robot]

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[Rishit-dagli]

On reading, I believe this change would be better suited to the OpenAPI specification since this is anyways being generated.

@roycaihw @yliaog Could you shed some light on contributing to the specification?