warning generated by _pad_category_counts() when no posts

[dougal]

If there are no posts in the db, _pad_category_counts() generates a warning due to a foreach over an empty array. Diff attached.

[dougal]

fix empty foreach warning

[foolswisdom]

Warning generated is:

Warning: Invalid argument supplied for foreach() in
/web/server/path/httpd.www/lions/wp-includes/category.php on line 297

[ryan]

(In [4716]) Cast to array to avoid warning. fixes #3556