Fix _get_data in CombinedDataset

[robin-p-schmitt]

Right now, CombinedDataset always returns an array of size (0,). With this fix, it returns an array of size (0, dim) instead, where dim is the expected dimension of the data key.

Fix #1772

[albertz]

Suggested change

	data_dim = self.data_dims[data_key]
	if data_dim[1] == 1:
	# sparse data
	return numpy.zeros((0,), dtype=self.data_dtypes[data_key])
	return numpy.zeros((0, data_dim[0]), dtype=self.data_dtypes[data_key])
	shape: List[Union[None, int]] = [0] * self.num_outputs[data_key][1]
	if shape and not self.is_data_sparse(data_key):
	shape[-1] = self.get_data_dim(data_key)
	return numpy.zeros(shape, dtype=self.data_dtypes[data_key])