Looks like there is an issue in the test. The content "should" never be empty/null but this is currently allowed in the code.
org.jboss.aerogear.diffsync.DiffSyncHandler - Doc:{"msgType":"add","id":"efc5393e-4de0-40f2-8334-f3c6c4c937c3","clientId":"client1","content":"Do or do not, there is no try."}
org.jboss.aerogear.diffsync.DiffSyncHandler - Doc:{"msgType":"add","id":"efc5393e-4de0-40f2-8334-f3c6c4c937c3","clientId":"client2"}
I'm adding an explicit check for this and will look into the test now.