Calendar import does not work if the iCal file has empty title events.

The import of an iCal file that contains an event with an empty summary will result in a false positive message (the import confirmation is received, but no event is imported).

Issue first reported on: https://git.xwikisas.com/xwiki-sas-cpt/mpi-for-biology-of-ageing/-/issues/4.

