BUG: msct_image:  re-added initialization of an empty header : fix a bug when getting the dimension of an image saved from a numpy array
BUG: msct_image:  re-added initialization of an empty header : fix a bug when getting the dimension of an image saved from a numpy array

Former-commit-id: 1890e3507082ac46ad7f696cea09e406e8bec9d6
Former-commit-id: 1432de0b3c853d564985a8f98422b3c2cb74898c
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox