Make SseConnection extend StreamChannelMixin<String> (#102)

I don't see any reason for the type argument to be nullable. Seems like a null safety migration artifact.

lib/src/server/sse_handler.dart

@@ -28,7 +28,7 @@ class _SseMessage {
 }
 
 /// A bi-directional SSE connection between server and browser.
-class SseConnection extends StreamChannelMixin<String?> {
+class SseConnection extends StreamChannelMixin<String> {
   /// Incoming messages from the Browser client.
   final _incomingController = StreamController<String>();