Chokes on numeric directory names

### Steps

1. Install the plugin.
2. `redirect_from: ['2024']`

### Expected

The `/2024/` URL path redirects.

### Actual

```
.bundle/ruby/2.7.0/gems/jekyll-3.9.3/lib/jekyll/filters/url_filters.rb:57:in `ensure_leading_slash': undefined method `empty?' for 2024:Integer (NoMethodError)
```