Save系のメソッドからチェーンできる必要はなさそうなので戻り値をvoidに fix

source/src/main/java/one/nem/lacerta/source/file/impl/FileManagerImpl.java

@@ -188,22 +188,18 @@ public class FileManagerImpl implements FileManager {
 
     @Override
     public void saveXml(Document document, String fileName) throws IOException {
-        return null;
     }
 
     @Override
     public void saveXml(Document document) throws IOException {
-        return null;
     }
 
     @Override
     public void saveBitmap(Bitmap bitmap, String fileName) throws IOException {
-        return null;
     }
 
     @Override
     public void saveBitmap(Bitmap bitmap) throws IOException {
-        return null;
     }
 
     @Override