[HDINSIGHT] NullRef SShCreds bug

[nemanja88]

Fixing a bug where a null ref was thrown when SSh is not provided for Linux clusters.

[azurecla]

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[azuresdkci]

Can one of the admins verify this patch?

[shahabhijeet]

@azuresdkci test this please

[shahabhijeet]

@azuresdkci test this please

[shahabhijeet]

@nemanja88 I don't see any test coverage for this case, as it seems like you fixed a null ref issue.

[markcowl]

@nemanja88 Please add test coverage to prevent regressions.

[markcowl]

@nemanja88 Please respond. We will have to close this PR at EOW if there is no response to review comments.

[markcowl]

@azuresdkci add to whitelist