Empty dataset throws MissingRequiredWarning

[rly]

I think we decided that an empty dataset should be allowed. So the following MissingRequiredWarning should not be thrown.

In /tests/unit/build_tests/test_io_map.py:

def test_build_empty(self):
        ''' Test default mapping functionality when no attributes are nested '''
        container = Bar('my_bar', [], 'value1', 10)
        builder = self.mapper.build(container, self.manager)
        expected = GroupBuilder('my_bar', datasets={'data': DatasetBuilder('data', [])},
                                attributes={'attr1': 'value1', 'attr2': 10})
        self.assertDictEqual(builder, expected)

results in:

c:\users\ryan\documents\nwb\hdmf\src\hdmf\build\map.py:974: MissingRequiredWarning: dataset 'data' for 'my_bar' of type (Bar)