COO-1118: Show an actionable error message if the namespace filter is empty

[andreasgerstmayr]

Show an actionable error message if the namespace filter is empty:

[openshift-ci-robot]

@andreasgerstmayr: This pull request references COO-1118 which is a valid jira issue.

Warning: The referenced jira issue has an invalid target version for the target branch this PR targets: expected the bug to target the "4.21.0" version, but no target version was set.

In response to this:

Show an actionable error message if the namespace filter is empty:

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the openshift-eng/jira-lifecycle-plugin repository.

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: andreasgerstmayr

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[openshift-ci]

@andreasgerstmayr: all tests passed!

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[andreasgerstmayr]

/label ux-approved
minimal UX changes

/label qe-approved
tiny change, no QE required

[openshift-ci-robot]

@andreasgerstmayr: This pull request references COO-1118 which is a valid jira issue.

Warning: The referenced jira issue has an invalid target version for the target branch this PR targets: expected the bug to target the "4.21.0" version, but no target version was set.

In response to this:

Show an actionable error message if the namespace filter is empty:

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the openshift-eng/jira-lifecycle-plugin repository.