Mapss T2 calculation requires mask[BUG]

[gattia]

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

https://github.com/ad12/DOSMA/blob/bd5efecbb944263c9a5d7853f154d9071c72ba62/dosma/scan_sequences/mri/mapss.py#L225-L232

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.