Null matrix parameters not accepted by the reasteasy client library

When you send a null for a matrix param on a proxy-ed request via the
client lib you get an NPE in the invoker.

java.lang.NullPointerException
at org.jboss.resteasy.client.ClientRequest.toString(ClientRequest.java:213)
at org.jboss.resteasy.client.ClientRequest.matrixParameter(ClientRequest.java:268)
at org.jboss.resteasy.client.core.marshallers.MatrixParamMarshaller.build(MatrixParamMarshaller.java:20)
at org.jboss.resteasy.client.core.ClientInvoker.invoke(ClientInvoker.java:98)
at org.jboss.resteasy.client.core.ClientProxy.invoke(ClientProxy.java:59)
at $Proxy23.getTickets(Unknown Source)

The other marshallers handle null, but MatrixParamMarshaller does not.