The `--sender-namespace` should be filled in from `--namespace` option, if given

[cardil]

The default value of --sender-namespace option should be equal to the value of a --namespace, if --namespace option is given. Otherwise, it should be equal to current context namespace.

/kind bug
/kind good-first-issue

[github-actions]

This issue is stale because it has been open for 90 days with no
activity. It will automatically close after 30 more days of
inactivity. Reopen the issue with /reopen. Mark the issue as
fresh by adding the comment /remove-lifecycle stale.

[cardil]

/remove-lifecycle stale
/reopen
/triage accepted

[knative-prow]

@cardil: Reopened this issue.

In response to this:

/remove-lifecycle stale
/reopen
/triage accepted

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[cardil]

/assign