Fix NPE when a null message comes in from a client

service/src/main/java/org/whispersystems/textsecuregcm/entities/IncomingMessageList.java

@@ -4,6 +4,8 @@
  */
 package org.whispersystems.textsecuregcm.entities;
 
+import com.fasterxml.jackson.annotation.JsonInclude;
+import com.fasterxml.jackson.annotation.JsonInclude.Include;
 import com.fasterxml.jackson.annotation.JsonProperty;
 
 import javax.validation.Valid;
@@ -15,6 +17,7 @@ public class IncomingMessageList {
   @JsonProperty
   @NotNull
   @Valid
+  @JsonInclude(Include.NON_NULL)
   private List<IncomingMessage> messages;
 
   @JsonProperty