Write VrtSecDecay containers for events without a PV

Hi,

This MR fixes a bug in the VrtSecDecay algorithm noticed in ATLASDPD-1548. The algorithm requires a primary vertex in the event, and returned before writing the containers. This caused a number of issues downstream. Writing empty containers before returning fixed the issues. The fix was tested on tasks that failed.

Tagging @cjsmith , @onofrio

Cheers, Matthew