Gracefully handle pdu null case in SMSDispatcher.sendMultipartText

In SMSDispatcher.sendMultipartText, if it has a problem to encode the SMS and get null pdu, the sentIntent will not be sent back to APP and phone process may crash.

Bug: 113640956
Test: make
      atest GsmSmsDispatcherTest#testSendMultipartTextWithInvalidText
      atest GsmSmsDispatcherTest

Change-Id: I7810875e018385af767c7fe2bc1ca776ccb83a6c