Fix SparseVector::insert(Index) assigning int to Scalar

Scalar doesn't necessarily support implicit construction from int or
assignment from int.

Here's the error message I got without this fix:
```
/home/tav/git/Sleipnir/build/_deps/eigen3-src/Eigen/src/SparseCore/SparseVector.h:180:25: error: no match for ‘operator=’ (operand types are ‘Eigen::internal::CompressedStorage<ExplicitDouble, int>::Scalar’ {aka ‘ExplicitDouble’} and ‘int’)
  180 |     m_data.value(p + 1) = 0;
      |     ~~~~~~~~~~~~~~~~~~~~^~~
```

See merge request libeigen/eigen!2046