[BUGFIX] Set property hreflang of SiteLanguage to empty string

By defining `$hreflang` of SiteLanguage to an empty string instead of
'en-US' wrong hreflang tags are avoided in the frontend.

Even though the field is set to required there are usecases where this
field is not set, e.g. by changing the site configuration manually. This
would have lead to hreflang tags pointing to a valid site but with a
wrong hreflang information.

Resolves: #92418
Releases: master
Change-Id: I042c275a2b107cac48a877c7e4043f69e378347d
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/65888


Tested-by: Richard Haeser <richard@richardhaeser.com>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Martin Kutschker <mkutschker-typo3@yahoo.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Richard Haeser <richard@richardhaeser.com>
Reviewed-by: Guido Schmechel <guido.schmechel@brandung.de>
Reviewed-by: Martin Kutschker <mkutschker-typo3@yahoo.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>