Crash when PeriodicEvent has no target

[dkachuma]

Describe the bug
PeriodicEvent with no target set causes a crash

To Reproduce
voidage.xml.gz
In that attached case I have a PeriodicEvent node that doesn't have a target. It's purpose is to contain other PeriodicEvent elements.

        <PeriodicEvent
            name="superEvent"
            timeFrequency="1">
            <PeriodicEvent
                name="solverApplications"
                target="/Solvers/EVENTTEST" />
            <PeriodicEvent
                name="vtkOutput"
                target="/Outputs/EVENTTEST" />
        </PeriodicEvent>

There appears to be a change that has caused this kind of setup to not work anymore. For instance, this used to work at 1a7dd3f but fails at 246e3a8.

Screenshots

MainGrid: importing field data from mesh dataset
   TableFunction: INITIALISATION_left_table
   TableFunction: INITIALISATION_right_table
   TableFunction: INITIALISATION_left_table
   TableFunction: INITIALISATION_right_table
Received signal 11: Segmentation fault

** StackTrace of 6 frames **
Frame 0: /lib64/libc.so.6
Frame 1: geos::PeriodicEvent::validate() const
Frame 2: geos::EventManager::run(geos::DomainPartition&)
Frame 3: geos::GeosxState::run()
Frame 4: main
Frame 5: __libc_start_main
Frame 6: /workrd/SCR/GEOSX/install/gcc8/GEOSX-246e3a8/bin/geosx
=====

Platform:

• Compiler: gcc 8.3.0
• GEOSX version: 0.2.0 (HEAD, sha1: 246e3a8)