Fix a NPE, Http20Draft12 requires non-null debugData for FrameWriter.goAway().

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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=74143304
diff --git a/core/src/main/java/com/google/net/stubby/newtransport/okhttp/OkHttpClientTransport.java b/core/src/main/java/com/google/net/stubby/newtransport/okhttp/OkHttpClientTransport.java
index 114e024..d9f3be7 100644
--- a/core/src/main/java/com/google/net/stubby/newtransport/okhttp/OkHttpClientTransport.java
+++ b/core/src/main/java/com/google/net/stubby/newtransport/okhttp/OkHttpClientTransport.java
@@ -152,7 +152,7 @@
       abort(new Status(Code.INTERNAL, "Transport stopped"));
       // Send GOAWAY with lastGoodStreamId of 0, since we don't expect any server-initiated streams.
       // The GOAWAY is part of graceful shutdown.
-      frameWriter.goAway(0, ErrorCode.NO_ERROR, null);
+      frameWriter.goAway(0, ErrorCode.NO_ERROR, new byte[0]);
     }
     stopIfNecessary();
   }