fix #962 - avoid NRE, and ensure that CreateBridge(cmd) can spawn connections if needed

src/StackExchange.Redis/ServerEndPoint.cs

@@ -178,7 +178,7 @@ public PhysicalBridge GetBridge(RedisCommand command, bool create = true)
                 case RedisCommand.PUNSUBSCRIBE:
                     return subscription ?? (create ? subscription = CreateBridge(ConnectionType.Subscription, null) : null);
                 default:
-                    return interactive;
+                    return interactive ?? (create ? interactive = CreateBridge(ConnectionType.Interactive, null) : null);
             }
         }
 
@@ -559,7 +559,7 @@ internal Task<T> WriteDirectAsync<T>(Message message, ResultProcessor<T> process
             var source = ResultBox<T>.Get(tcs);
             message.SetSource(processor, source);
             if (bridge == null) bridge = GetBridge(message.Command);
-            var result = bridge.TryWrite(message, isSlave);
+            var result = bridge?.TryWrite(message, isSlave) ?? WriteResult.NoConnectionAvailable;
             if (result != WriteResult.Success)
             {
                 var ex = Multiplexer.GetException(result, message, this);