Optimization: Don't repeatedly check unchanging value

[bookwyrm]

https://github.com/leonstafford/wp2static/blob/84f1d33bcadbe0a7c485a72c8528b5703f147454/src/FilesHelper.php#L60

The value of $site_path doesn't change while we iterate over files in the filesystem, but we're checking the value each time.

Since if we have a non-string value for $site_path we won't ever get any files, it would be better to pull this check out before the iterator an simply short-circuit the iterator and return an empty array if $site_path is not a string.

[leonstafford]

💯