Do not crash when internal files are in accessible

We do not actually need access to the internal files, so simply skip
migration if there doesn't appear to be an internal folder.

android-project/app/src/main/java/org/diasurgical/devilutionx/DevilutionXSDLActivity.java

@@ -160,7 +160,10 @@ public class DevilutionXSDLActivity extends SDLActivity {
 	}
 
 	private void migrateSaveGames() {
-		for (File internalFile : Objects.requireNonNull(getFilesDir().listFiles())) {
+		File[] files = getFilesDir().listFiles();
+		if (files == null)
+			return;
+		for (File internalFile : files) {
 			migrateFile(internalFile);
 		}
 	}