use Proxy.NO_PROXY instead of null value for no proxy

OpenKeychain/src/main/java/org/sufficientlysecure/keychain/util/ParcelableProxy.java

@@ -17,13 +17,14 @@
 
 package org.sufficientlysecure.keychain.util;
 
-import android.os.Parcel;
-import android.os.Parcelable;
-import android.support.annotation.Nullable;
 
 import java.net.InetSocketAddress;
 import java.net.Proxy;
 
+import android.os.Parcel;
+import android.os.Parcelable;
+import android.support.annotation.NonNull;
+
 /**
  * used to simply transport java.net.Proxy objects created using InetSockets between services/activities
  */
@@ -48,10 +49,10 @@ public class ParcelableProxy implements Parcelable {
         return new ParcelableProxy(null, -1, null);
     }
 
-    @Nullable
+    @NonNull
     public Proxy getProxy() {
         if (mProxyHost == null) {
-            return null;
+            return Proxy.NO_PROXY;
         }
         /*
         * InetSocketAddress.createUnresolved so we can use this method even in the main thread

OpenKeychain/src/main/java/org/sufficientlysecure/keychain/util/Preferences.java

@@ -23,6 +23,7 @@ import android.content.SharedPreferences;
 
 import android.content.res.Resources;
 import android.preference.PreferenceManager;
+import android.support.annotation.NonNull;
 import android.support.annotation.Nullable;
 
 import org.sufficientlysecure.keychain.Constants;
@@ -325,6 +326,7 @@ public class Preferences {
             else this.parcelableProxy = new ParcelableProxy(hostName, port, type);
         }
 
+        @NonNull
         public Proxy getProxy() {
             return parcelableProxy.getProxy();
         }