[BUGFIX] Fix sql error in EXT:linkvalidator

If no page is selected, skip sql queries in LinkAnalyzer
getLinkStatistics.

Resolves: #77588
Releases: master, 7.6, 6.2
Change-Id: I3b45cbad6962c5327d860b182e61255018bf0e53
Reviewed-on: https://review.typo3.org/49547


Tested-by: Bamboo TYPO3com <info@typo3.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>