client.baseRetryPolicy check if response is nil before accessing attributes

[tty2]

baseRetryPolicy is accessable from public DefaultRetryPolicy and ErrorPropagatedRetryPolicy and it is called without any checks.
There is possible case with call this function with passing nil *http.Response inside and catch panic.
With this changes they get error with explanation.

[hashicorp-cla]

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[manicminer]

@tty2 Thanks for submitting this PR. I agree we should be nil-checking the response, however I'd suggest that the request should be retried in that circumstance, since a nil response is often attributed to the connection being closed or the server misbehaving - both temporary conditions that IMO we should smooth over.