Fix nans in gfa

I've been wanting to fix this for months, I just never got to it. `fit.gfa` not returns 0 instead of nan when all the SH coefficients are 0 (almost exclusively in masked voxels). This should also get rid of one of the annoying warnings during the tests :).
The sun being equal to zero is not the same as all the coefficients being
equal to zero. That is the part that confused me.