[BUGFIX] Render favicon tag only if file exists

The favicon defined by `page.shortcutIcon`
creates a <link rel="icon"> tag in the frontend.

This tag should only be renderd if the defined
icon exists.

Resolves: #24481
Releases: master, 6.2
Change-Id: Ia266c730c0ea517b03d6666502b68f41efaf1304
Reviewed-on: http://review.typo3.org/35157
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>