Need to check for null when creating Tags.

[athipp]

If a server comesback with null for the Tags object. then the Get comand will fail creating a dictionary object with a null reference.

Fix is to check for null and use a different constructor instead.

Same for sku

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[cormacpayne]

@athipp please re-target this PR to the dev branch

[markcowl]

@athipp please add test coverage, as we discussed

[athipp]

@markcowl This was a bug in the New-AzureRMAnalyis... API. The existing tests were actually sufficient to catch the scenario except that the New- API was never creating the server with null 'Tag'..so the Get- api in the test always got an empty dictionary. For servers created via Ibiza outside of powershell the Get api would return 'null' tag exposing the bug...the Fix was in Create- Update codepath.