fixing the build error

Author: Sophia Tevosyan

client/src/main/java/com/microsoft/durabletask/DurableTaskGrpcClient.java

@@ -63,6 +63,7 @@ public final class DurableTaskGrpcClient extends DurableTaskClient {
 
     DurableTaskGrpcClient(int port) {
         this.dataConverter = new JacksonDataConverter();
+        this.defaultVersion = null;
 
         // Need to keep track of this channel so we can dispose it on close()
         this.managedSidecarChannel = ManagedChannelBuilder