URL Details: Avoid PHP notice when parsing protocol-relative icon URLs

[Mamaduka]

Description

Sites might use protocol-relative URLs for icons. When a similar URL is parsed with parse_url, it doesn't return schema and triggered PHP notice.

Example: https://3v4l.org/SFEiZ

How has this been tested?

1. Create a post.
2. Add a paragraph with a link to https://twitter.com/
3. Display rich URL preview.
4. WP shouldn't log the PHP Notice: Undefined index: scheme message.

Types of changes

Bugfix

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).

[getdave]

Nice catch thanks!