runner: replace '.size() == 0' with '.isEmpty()' (collections)

http://docs.oracle.com/javase/1.5.0/docs/api/java/util/ArrayList.html#isEmpty()

Change-Id: I9a8f4feab9120d2d9fbf845f305380cda9b263bb
Reviewed-on: https://gerrit.libreoffice.org/11968
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>