fix unexpected ValueError

[jimmieliu]

When value is a numpy array, comparing a numpy array with None object will give another numpy array. Then a ValueError will be raised, says: ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all(). For example, you can test this funtion with value = numpy.array([1,2,3]), and default=None.

Thanks for your contribution!

If you're sending a large PR (e.g., >50 lines), please open an issue first about the feature/bug, and indicate how you want to contribute.

Use contributing guidelines before opening up the PR to follow MMF style guidelines.