torch.sparse.softmax avoid div by zero and invalid kernel launch parameters (#82149)
### Description
Small changes needed to deal with nnz 0 inputs.
### Issue
https://github.com/pytorch/pytorch/issues/82107
### Testing
Added additional test coverage to reproduce bug reported in issue. Tested resulting values by conversion `to_dense`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/82149
Approved by: https://github.com/jbschlosser, https://github.com/ezyang
