Support deleting custom claims by passing None

[hiranya911]

In #320 we removed the ability to delete user attributes by passing None to auth.update_user(). This change inadvertently affected the auth.set_custom_user_claims() method which should support deleting claims via passing None. This caused an integration test failure:

_______________________________________________________________________________ test_update_custom_user_claims _______________________________________________________________________________

new_user = <firebase_admin._user_mgt.UserRecord object at 0x7f876c0e1d10>

    def test_update_custom_user_claims(new_user):
        assert new_user.custom_claims is None
        claims = {'admin' : True, 'package' : 'gold'}
        auth.set_custom_user_claims(new_user.uid, claims)
        user = auth.get_user(new_user.uid)
        assert user.custom_claims == claims
    
        claims = {'admin' : False, 'subscription' : 'guest'}
        auth.set_custom_user_claims(new_user.uid, claims)
        user = auth.get_user(new_user.uid)
        assert user.custom_claims == claims
    
        auth.set_custom_user_claims(new_user.uid, None)
        user = auth.get_user(new_user.uid)
>       assert user.custom_claims is None
E       AssertionError: assert {'admin': False, 'subscription': 'guest'} is None
E        +  where {'admin': False, 'subscription': 'guest'} = <firebase_admin._user_mgt.UserRecord object at 0x7f876c01b590>.custom_claims

../integration/test_auth.py:308: AssertionError

This PR fixes the set_custom_user_claims() method, which now supports removing claims via both None and DELETE_ATTRIBUTE. I also updated the update_user() integration test to use DELETE_ATTRIBUTE instead of None.

[lahirumaramba]

LGTM!