arrow

GH-45119: [Ruby] Use #size for Arrow::Column#size backend (#45133)

### Rationale for this change

`#n_rows` is available only on `Arrow::ChunkedArray`. It's not available on `Arrow::Array`.

### What changes are included in this PR?

It's available on both of `Arrow::Array` and `Arrow::ChunkedArray`.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes.
* GitHub Issue: #45119

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>