Support zero-length `axis` in `.map_axis/_mut()`

The implementation of `.map_axis/_mut()` panics when the specified `axis` is zero-length. Ideally, we'd just apply `mapping` to empty views instead.

Example:

```rust
extern crate ndarray;

use ndarray::prelude::*;

fn main() {
    let a = Array2::<f32>::zeros((0, 2));
    a.map_axis(Axis(0), |_| {});
}
```