Return an error on nil response

[jhendrixMSFT]

Properly handle the case where the HTTP client returns a nil response
and error.
Refactored poller.Poll() to simplify error propagation.

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[jhendrixMSFT]

Fixes #14729

[jhendrixMSFT]

There was a fix in azcore to return an error if the response and error were both nil. It could be the case that the repro was using an older version of azcore without the fix.