Mapss T2 calculation requires mask[BUG]

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

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.

	# 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
	)