Fix returning invalid strides and dimensions for rank zero arrays.

[adamreichold]

The docs contain:

When nd is 0, the array is sometimes called a rank-0 array. Such arrays have undefined dimensions and strides and cannot be accessed.

[adamreichold]

The test has funny failures like

thread 'rank_0_array_has_invalid_strides_dimensions' panicked at 'assertion failed: `(left == right)`
  left: `[]`,
 right: `[]`', tests/array.rs:77:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

without the fix as p == ptr::null() in this case which is not valid for slice::from_raw_parts even when n == 0.

[davidhewitt]

Nice catch 👍