Internal Http SystemProxyInfo object should never be null (dotnet/corefx#37306)

Internal Http SystemProxyInfo object should never be null (dotnet/corefx#37306)

In order to provide a consistent experience with the HttpClient IWebProxy objects,
the returned internal proxy object which represent the system/platform proxy settings
should never be null. If the platform's settings indicate that no proxy is being used,
then return an instance of the internal HttpNoProxy object.

Note that even if the platform settings indicate that a proxy could be used, any
particular Http request might still not go thru a proxy. The final determination of
what proxy is being used for a request is still governed by the return of the
IWebProxy.IsBypassed and IWebProxy.GetProxy methods.

Contributes to dotnet/corefx#36553

Commit migrated from https://github.com/dotnet/corefx/commit/830bf7fc0346d9453d85ad4ec726c92ac3e72998