Add test for active with no DropoutMask defined

[eileen-kuehn]

Currently the function active statically builds on the existence of DropoutMask to return the number of active gates in a circuit.

masKIT/maskit/_masked_circuits.py

Lines 159 to 165 in fe7dee9

	def active(self) -> int:
	"""
	Number of active gates in the circuit based on layer, wire, and parameter mask.
	Entangling gates are not included.
	"""
	mask = self.full_mask(mask_type=DropoutMask)
	return mask.size - np.sum(mask)

During handling of edge cases in full_mask this issue is properly fixed, but might not be guaranteed in the future.

masKIT/maskit/_masked_circuits.py

Lines 132 to 144 in fe7dee9

	def full_mask(self, mask_type: Type[Mask]) -> np.ndarray:
	"""
	Accumulated mask of layer, wire, and parameter masks for a given type of Mask.
	Note that this mask is readonly.
	"""
	if Axis.PARAMETERS in self.masks and mask_type in self.masks[Axis.PARAMETERS]:
	result = self.masks[Axis.PARAMETERS][mask_type].mask.copy()
	else:
	result = np.zeros(
	self.parameters.shape,
	dtype=mask_type.elemental_type,
	requires_grad=False,
	)

To properly handle this, we should implement a proper unit test.