STY: Annotate DWI optional data class parameters as such

[jhlegarreta]

Annotate DWI optional data class parameters as such: add None to the type annotation of bzero and eddy_xfms attributes.

Ignore the bzero indexing static checking error when serializing the data to NIfTI: the indexing operation is protected by a conditional no_bzero statement, so at that point the class instance is bound to have a non-None bzero attribute value.

Assert that bzero is not None in tests to avoid static type checking errors.

[jhlegarreta]

No else exists in

nifreeze/src/nifreeze/data/dmri.py

Lines 390 to 401 in d27ba75

	if b0_file:
	b0img = load_api(b0_file, SpatialImage)
	b0vol = np.asanyarray(b0img.dataobj)
	# We'll assume your DWI class has a bzero: np.ndarray | None attribute
	dwi_obj.bzero = b0vol
	# Otherwise, if any volumes remain outside gradmsk, compute a median B0:
	elif np.any(~gradmsk):
	# The b=0 volumes are those that did NOT pass b0_thres
	b0_volumes = fulldata[..., ~gradmsk]
	# A simple approach is to take the median across that last dimension
	# Note that axis=3 is valid only if your data is 4D (x, y, z, volumes).
	dwi_obj.bzero = np.median(b0_volumes, axis=3)

So bzero is allowed to be None.