Skip user_id when it is null in PusherEvent constructor

Adding a null user_id stores a JsonNull, which makes getUserId() throw
UnsupportedOperationException. Decrypted events on private-encrypted
channels pass a null user ID, so they would hit this.

Author: Keith-wright

src/main/java/com/pusher/client/channel/PusherEvent.java

@@ -70,7 +70,9 @@ public String toString() {
     public PusherEvent(String event, String channel, String userId, String data) {
         jsonObject.addProperty("event", event);
         jsonObject.addProperty("channel", channel);
-        jsonObject.addProperty("user_id", userId);
+        if (userId != null) {
+            jsonObject.addProperty("user_id", userId);
+        }
         jsonObject.addProperty("data", data);
     }
 

src/test/java/com/pusher/client/channel/PusherEventTest.java

@@ -1,6 +1,7 @@
 package com.pusher.client.channel;
 
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNull;
 
 import org.junit.Before;
 import org.junit.Test;
@@ -45,6 +46,19 @@ public void testUserIdIsExtracted() {
         );
         assertEquals("my-user-id", e.getUserId());
     }
+
+    @Test
+    public void testUserIdIsSetByConstructor() {
+        final PusherEvent e = new PusherEvent("my-event", "my-channel", "my-user-id", "{}");
+        assertEquals("my-user-id", e.getUserId());
+    }
+
+    @Test
+    public void testNullUserIdFromConstructorIsNull() {
+        final PusherEvent e = new PusherEvent("my-event", "my-channel", null, "{}");
+        assertNull(e.getUserId());
+    }
+
     @Test
     public void testErrorData() {
         final PusherEvent e = PusherEvent.fromJson(