Group grey matter template cover voxels with no signal for a subject

[htwangtw]

When using a group grey matter template, it's possible that the mask cover voxels with no signal for a subject. In that case, the inverse transform function of NiftiMasker will fail.

https://github.com/SIMEXP/giga_connectome/blob/916e15533c59e8e8bf2e5cd54077b7ea677d2e52/giga_connectome/denoise.py#L133

Two potential solutions:

1. fill NaN values in with zeros so when reconstructing the nifti, the voxel will be treated as background.
2. Skip this subject and keep with workflow running - if going for this option, this should also apply to other known issues (cosine and compcor labelling related issues)