[BUGFIX] Generate absolute url when the site base is only a slash

[RinyVT]

Summary

This PR can be summarized in the following changelog entry:

• When the base in site configuration is only /, the absolute URL was not generated in the backend. This fix always provides a full url.

Relevant technical choices:

• When the generated URL has a slash at the first position, the TYPO3_REQUEST_HOST is prepended.

Test instructions

This PR can be tested by following these steps:

• Set your site base to / and see if the snippet preview in the page module and page properties still shows results.

Quality assurance

• I have tested this code to the best of my abilities
• I have added unittests to verify the code works as intended

Fixes #279