Handled the close message in UnifiedChannelizer

The UnifiedChannelizer was generating a lot of junk validation errors and logging because the close message was removed at 3.5.0 and it was not accounted for here. While the UnifiedChannelizer doesn't do anything particular with the close message it did need to account for it during the messages validation to avoid these errors CTR

gremlin-server/src/main/java/org/apache/tinkerpop/gremlin/server/handler/UnifiedHandler.java

@@ -209,8 +209,10 @@ protected void channelRead0(final ChannelHandlerContext ctx, final RequestMessag
     }
 
     protected void validateRequest(final RequestMessage message, final GraphManager graphManager) throws SessionException {
-        if (!message.optionalArgs(Tokens.ARGS_GREMLIN).isPresent()) {
-            final String msg = String.format("A message with an [%s] op code requires a [%s] argument.", message.getOp(), Tokens.ARGS_GREMLIN);
+        // close message just needs to be accounted for here as of 3.5.2. it will not contain a "gremlin" arg. unified
+        // channelizer basically ignores the close message otherwise
+        if (!message.getOp().equals(Tokens.OPS_CLOSE) && !message.optionalArgs(Tokens.ARGS_GREMLIN).isPresent()) {
+            final String msg = String.format("A message with a [%s] op code requires a [%s] argument.", message.getOp(), Tokens.ARGS_GREMLIN);
             throw new SessionException(msg, ResponseMessage.build(message).code(ResponseStatusCode.REQUEST_ERROR_INVALID_REQUEST_ARGUMENTS).statusMessage(msg).create());
         }