BUG: do not normalize intensity if slice is GM in slice is empty
BUG: do not normalize intensity if slice is GM in slice is empty

Former-commit-id: 585e3df2193772ddc778fdc90225c6e05264bdf3 [formerly dac1c19217d7ccffb88846731f26de17d6fc468c]
Former-commit-id: 6266d4b786ced5f65f37c322da6395fe85d7dbfd
Former-commit-id: 142a7290baaedefb471bdc935a5409ca10f997ae
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox