Adding extra check for Katib core namespace env

[johnugeorge]

If DefaultKatibNamespaceEnvName is not set, it will use default namespace. This will be used in unit tests where env is not set.
/hold
Fixes: #604

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[gaocegege]

LGTM 👍
/lgtm

[johnugeorge]

/hold

[johnugeorge]

Closing this as web hooks will be disabled in unit tests after #657
/close

[k8s-ci-robot]

@johnugeorge: Closed this PR.

In response to this:

Closing this as web hooks will be disabled in unit tests after #657
/close

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