PHP 7.1 error in wp_star_rating()

[afragen]

In PHP 7.1, passing a null or empty value to round() results in a PHP warning on the Add Plugin page. This happens when the star rating is 0.

This from line 2121 from /wp-admin/includes/template.php .

[SergeyBiryukov]

Apparently $rating can be a float (0..5 in .5 increments) if $r['type'] is rating. Core only uses percent type, but float usage should also be documented.

I could not reproduce the actual warning with '0' as $r['rating']. Reproduced with '', but the API seems to return the former. Seems like a good fix anyway.

[SergeyBiryukov]

In 41184:

Plugins: In wp_star_rating(), use explicit type casting for $rating to avoid a "non-numeric value encountered" warning in PHP 7.1.

Clarify in the function DocBlock that $rating can be a float.

Props afragen.
Fixes #41484.