[BUGFIX] typolink: Ensure explode() is using string

Link to home page with section and enabled realurl ends up in uncaught
exception because the new PageLinkBuilder is a PHP7 strict
class, and thus, throws errors when explode() is used with null
as parameter.

Releases: master
Resolves: #80664
Change-Id: I206c4846ca9b381cca85d99ba3cc846dadbb266c
Reviewed-on: https://review.typo3.org/52344


Reviewed-by: Claus Due <claus@phpmind.net>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>