Do not expose `HttpResponseMessage.Content == null` to user code

[dougbu]

In recent (5.0 and newer) .NET runtimes, Content==null is impossible for user code to see. Even in older runtimes, HttpClient would never leave Content==null either.

• see dotnet/runtime@b48900f for the 5.0 change

Public code in this repo both exposes Content==null (inconsistently) and expects (mostly in tests) to see that state. Bad enough we expose that state but it being inconsistent (depending on the runtime version) makes things worse.

The action here is to change our code to both remove the inconsistencies i.e., have Content!=null everywhere regardless of the runtime version and update our expectations to handle Content!=null. This includes reverting tests removed on net6.0 in #384.

[dougbu]

Anything to add @Tratcher or @stephentoub❔

[stephentoub]

Sounds fine.