Return null for getSubmitPdu() for encoding failure instead of NPE

It returns null as API description for null message or null destination
instead of NPE.

Bug: 111731144
Test: atest SmsMessageTest

Change-Id: I1dc8537b6d07a43345c266122a1c77e73e58e7e8
Signed-off-by: Taesu Lee <taesu82.lee@samsung.com>

telephony/java/android/telephony/SmsMessage.java

@@ -627,7 +627,7 @@ public class SmsMessage {
                     destinationAddress, message, statusReportRequested);
         }
 
-        return new SubmitPdu(spb);
+        return spb != null ? new SubmitPdu(spb) : null;
     }
 
     /**
@@ -655,7 +655,7 @@ public class SmsMessage {
                     destinationAddress, destinationPort, data, statusReportRequested);
         }
 
-        return new SubmitPdu(spb);
+        return spb != null ? new SubmitPdu(spb) : null;
     }
 
     // TODO: SubmitPdu class is used for SMS-DELIVER also now. Refactor for SubmitPdu and new