Check on Default Social Image asset url

[alexjcollins]

Description

The recent update to SEO 3.3.1 (Craft Pro 3.0.23.1) took out/crashed the front-end of one our Craft sites. Together with the guys at Craft we identified this being due to a bug in the SEO plugin (related to the ‘Default Social Image’):

Asset volumes have a setting which determines whether or not they are supposed to have public URLs. If you try to access a URL of an asset uploaded to such a volume, it will return an URL, which is a likely scenario here.

The plugin does not check whether the URL for an asset is a string or null (even though the docblock documents this behavior, https://github.com/craftcms/cms/blob/develop/src/elements/Asset.php#L706), before passing it on to a method which requires string.