Given how short the method is, maybe this is even shorter:

 File file = new File(getDirectory(), filename);
 try {
    byte[] raw = IO.readFully(file);
    return 0 < raw.length ? raw : null;
  } catch (FileNotFoundException notFound) {
    return null;
  }

On something this tiny, it isn't really worth trying to hoist the if outside of the try/catch.