inv of singular matrix returns some matrix without error

I tried this script.

```
require "numo/linalg"

a = Numo::DFloat[[0, 0],
                 [0, 0]]
p(Numo::Linalg.inv(a))
```

Result is here.

```
Numo::DFloat#shape=[2,2]
[[0, 0], 
 [0, 0]]
```

I think that a matrix with all zero elements has no inverse matrix.