[BUGFIX] Make rendering of priority option optional in XML sitemaps

According to https://www.sitemaps.org/de/protocol.html the option
`priority` is optional. If a SitemapProvider doesn't provide a priority
information the tag must be avoided to have a valid XML structure.

Resolves: #91286
Releases: master, 9.5
Change-Id: I0bd4379633359321ad39726e4357773880c15638
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64397


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Christian Eßl <indy.essl@gmail.com>
Tested-by: Josef Glatz <josefglatz@gmail.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Richard Haeser <richard@maxserv.com>
Reviewed-by: Christian Eßl <indy.essl@gmail.com>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Josef Glatz <josefglatz@gmail.com>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Richard Haeser <richard@maxserv.com>