[BUGFIX] PSR-7 Uri implementation must return scheme as string

TYPO3's PSR-7 implementation has a small flaw when having URLs
without a scheme, which should always return an empty string.

This change initializes the scheme with an empty string by default.

Resolves: #94872
Releases: master, 10.4
Change-Id: If2898d697f37e6e5fc8c4553d0bb500f7fc8b47e
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/70547


Tested-by: Benjamin Franzke <bfr@qbus.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Benjamin Franzke <bfr@qbus.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>