[BUGFIX] Don't try to render icon for empty identifier

In case a site is missing a proper site
configuration - having no default language
a fallback SiteLanguage object is used in
the page module. This fallback however does
provide an empty string as flag identifier,
which does no longer result in a broken icon.

Resolves: #100799
Releases: main, 12.4, 11.5
Change-Id: I23f3d9347d2ea8a63520086054b9fa842a9f5cfd
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78929


Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Jochen <rothjochen@gmail.com>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: Daniel Haupt <mail@danielhaupt.de>
Reviewed-by: Daniel Haupt <mail@danielhaupt.de>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Jochen <rothjochen@gmail.com>