Mapss T2 calculation requires mask[BUG]

[gattia]

Describe the bug
A clear and concise description of what the bug is.

DOSMA/dosma/scan_sequences/mri/mapss.py

Lines 225 to 232 in bd5efec

	# only calculate for focused region if a mask is available, this speeds up computation
	mask = tissue.get_mask()
	if mask_path is not None:
	mask = (
	fio_utils.generic_load(mask_path, expected_num_volumes=1)
	if isinstance(mask_path, (str, os.PathLike))
	else mask_path
	)

The above section of code shows that the tissue class is required for the T2 generation to go through. If the tissue class is not present then .get_mask() will fail (None type does not have the function). However, tissue class is meant to be optional.