add null check to onResult callback

[Cnilton]

PR to fix android crashing on result being null:

E FATAL EXCEPTION: main Process: com.poc_speech_recognigtion, PID: 11881 java.lang.NullPointerException: Attempt to invoke virtual method 'java.util.Iterator java.util.ArrayList.iterator()' on a null object reference at com.wenkesj.voice.VoiceModule.onResults(VoiceModule.java:354) at android.speech.SpeechRecognizer$InternalRecognitionListener$1.handleMessage(SpeechRecognizer.java:1062) at android.os.Handler.dispatchMessage(Handler.java:106) at android.os.Looper.loopOnce(Looper.java:230) at android.os.Looper.loop(Looper.java:319) at android.app.ActivityThread.main(ActivityThread.java:8893) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:608) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1103)