Improve test coverage for for x-site configuration parsing

17:22:41,798 WARN  [org.infinispan.remoting.transport.jgroups.CommandAwareRpcDispatcher] (Incoming-2,shared=tcp) ISPN000071: Caught exception when handling command SingleRpcCommand{cacheName='importantCache', command=PutKeyValueCommand{key=1, value=[B@363f983f, flags=null, putIfAbsent=false, metadata=MimeMetadata(contentType=text/plain), successful=true, ignorePreviousValue=false}}: java.lang.NullPointerException
        at org.infinispan.xsite.BackupReceiverRepositoryImpl.isBackupForRemoteCache(BackupReceiverRepositoryImpl.java:112) [infinispan-core-6.0.0.Alpha4.jar:6.0.0.Alpha4]
        at org.infinispan.xsite.BackupReceiverRepositoryImpl.getBackupCacheManager(BackupReceiverRepositoryImpl.java:95) [infinispan-core-6.0.0.Alpha4.jar:6.0.0.Alpha4]
        at org.infinispan.xsite.BackupReceiverRepositoryImpl.handleRemoteCommand(BackupReceiverRepositoryImpl.java:67) [infinispan-core-6.0.0.Alpha4.jar:6.0.0.Alpha4]
        at org.infinispan.remoting.transport.jgroups.CommandAwareRpcDispatcher.executeCommandFromRemoteSite(CommandAwareRpcDispatcher.java:234) [infinispan-core-6.0.0.Alpha4.jar:6.0.0.Alpha4]
        at org.infinispan.remoting.transport.jgroups.CommandAwareRpcDispatcher.handle(CommandAwareRpcDispatcher.java:209) [infinispan-core-6.0.0.Alpha4.jar:6.0.0.Alpha4]
        at org.jgroups.blocks.RequestCorrelator.handleRequest(RequestCorrelator.java:460) [jgroups-3.4.0.Alpha1.jar:3.4.0.Alpha1]
        at org.jgroups.blocks.RequestCorrelator.receiveMessage(RequestCorrelator.java:377) [jgroups-3.4.0.Alpha1.jar:3.4.0.Alpha1]
        at org.jgroups.blocks.RequestCorrelator.receive(RequestCorrelator.java:247) [jgroups-3.4.0.Alpha1.jar:3.4.0.Alpha1]
        at org.jgroups.blocks.MessageDispatcher$ProtocolAdapter.up(MessageDispatcher.java:665) [jgroups-3.4.0.Alpha1.jar:3.4.0.Alpha1]
        at org.jgroups.blocks.mux.MuxUpHandler.up(MuxUpHandler.java:130) [jgroups-3.4.0.Alpha1.jar:3.4.0.Alpha1]
        at org.jgroups.JChannel.up(JChannel.java:719) [jgroups-3.4.0.Alpha1.jar:3.4.0.Alpha1]
        at org.jgroups.stack.ProtocolStack.up(ProtocolStack.java:1002) [jgroups-3.4.0.Alpha1.jar:3.4.0.Alpha1]
        at org.jgroups.protocols.relay.RELAY2.deliver(RELAY2.java:612) [jgroups-3.4.0.Alpha1.jar:3.4.0.Alpha1]
        at org.jgroups.protocols.relay.RELAY2.route(RELAY2.java:508) [jgroups-3.4.0.Alpha1.jar:3.4.0.Alpha1]
        at org.jgroups.protocols.relay.RELAY2.handleMessage(RELAY2.java:483) [jgroups-3.4.0.Alpha1.jar:3.4.0.Alpha1]
        at org.jgroups.protocols.relay.RELAY2.handleRelayMessage(RELAY2.java:461) [jgroups-3.4.0.Alpha1.jar:3.4.0.Alpha1]
        at org.jgroups.protocols.relay.Relayer$Bridge.receive(Relayer.java:263) [jgroups-3.4.0.Alpha1.jar:3.4.0.Alpha1]
        at org.jgroups.JChannel.up(JChannel.java:749) [jgroups-3.4.0.Alpha1.jar:3.4.0.Alpha1]
        ...