add @Nullable annotation to outoging mms api

Closes #2830

// FREEBIE

src/org/thoughtcrime/securesms/mms/OutgoingLegacyMmsConnection.java

@@ -21,6 +21,7 @@ import android.net.ConnectivityManager;
 import android.net.NetworkInfo;
 import android.net.Uri;
 import android.support.annotation.NonNull;
+import android.support.annotation.Nullable;
 import android.util.Log;
 
 import org.apache.http.Header;
@@ -71,7 +72,7 @@ public class OutgoingLegacyMmsConnection extends LegacyMmsConnection implements
   }
 
   @Override
-  public SendConf send(@NonNull byte[] pduBytes) throws UndeliverableMessageException {
+  public @Nullable SendConf send(@NonNull byte[] pduBytes) throws UndeliverableMessageException {
     try {
       MmsRadio radio = MmsRadio.getInstance(context);
 

src/org/thoughtcrime/securesms/mms/OutgoingLollipopMmsConnection.java

@@ -22,6 +22,7 @@ import android.content.Intent;
 import android.os.Build.VERSION;
 import android.os.Build.VERSION_CODES;
 import android.support.annotation.NonNull;
+import android.support.annotation.Nullable;
 import android.telephony.SmsManager;
 import android.util.Log;
 
@@ -58,7 +59,7 @@ public class OutgoingLollipopMmsConnection extends LollipopMmsConnection impleme
 
   @Override
   @TargetApi(VERSION_CODES.LOLLIPOP)
-  public synchronized SendConf send(@NonNull byte[] pduBytes) throws UndeliverableMessageException {
+  public @Nullable synchronized SendConf send(@NonNull byte[] pduBytes) throws UndeliverableMessageException {
     beginTransaction();
     try {
       MmsBodyProvider.Pointer pointer = MmsBodyProvider.makeTemporaryPointer(getContext());

src/org/thoughtcrime/securesms/mms/OutgoingMmsConnection.java

@@ -1,11 +1,12 @@
 package org.thoughtcrime.securesms.mms;
 
 import android.support.annotation.NonNull;
+import android.support.annotation.Nullable;
 
 import org.thoughtcrime.securesms.transport.UndeliverableMessageException;
 
 import ws.com.google.android.mms.pdu.SendConf;
 
 public interface OutgoingMmsConnection {
-  SendConf send(@NonNull byte[] pduBytes) throws UndeliverableMessageException;
+  @Nullable SendConf send(@NonNull byte[] pduBytes) throws UndeliverableMessageException;
 }