Fix crashes on empty auth responses and repeated reconnects after sign-in

[Keith-wright]

What does this PR do?

Fixes two crashes where an exception escaped on the library's event thread.

• Closes NullPointerException when authorizing private channel subscription #372, which was first reported in NullPointerException when authorizing private channel #357. If a ChannelAuthorizer returned null or an empty string, subscribing to a private, presence or private-encrypted channel threw a NullPointerException instead of an AuthorizationFailureException. Gson returns null for that input, and the code called a method on the result. User sign-in gets the same check.
• Closes IllegalStateException error is thrown when the server is closed on a logged-in server #367. After a user signed in, a second reconnect attempt threw an IllegalStateException. The first attempt cleared the user ID but left the server-to-user channel marked as subscribed, so the second attempt asked for the channel name without a user ID. The channel is now marked unsubscribed, and the cleanup only runs while there's a user ID.

It also adds tests for each case.

CHANGELOG

• [FIXED] A ChannelAuthorizer that returns null or an empty string now causes an AuthorizationFailureException instead of a crash (NullPointerException when authorizing private channel subscription #372).
• [FIXED] Repeated reconnect attempts after user sign-in no longer crash with an IllegalStateException (IllegalStateException error is thrown when the server is closed on a logged-in server #367).