Avoid app crash on android when using nfc within a service

Change-Id: I28a165c7b7503b1c8159dd0c46f3f7450e599c74
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>

src/android/nfc/src/org/qtproject/qt5/android/nfc/QtNfc.java

@@ -153,6 +153,8 @@ public class QtNfc
 
     static public boolean isAvailable()
     {
+        if (m_activity == null) return false;
+
         m_adapter = NfcAdapter.getDefaultAdapter(m_activity);
         if (m_adapter == null) {
             //Log.e(TAG, "No NFC available (Adapter is null)");