[BUGFIX] Check for empty select icon setting in TcaMigration

The migrateExtAndSysextPathToEXTPath() method in the TcaMigration
checks if the icon path setting is empty before building the
paths.

Resolves: #69707
Releases: master
Change-Id: I8e1c37187da825459c5392f4115cb9ab75b99cc8
Reviewed-on: http://review.typo3.org/43224


Tested-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Michael Oehlhof <typo3@oehlhof.de>
Reviewed-by: Nicole Cordes <typo3@cordes.co>
Tested-by: Nicole Cordes <typo3@cordes.co>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>