core:Cannot add external event if flood rate is...

Cannot add external event if flood rate is not passed explicitly

Fixing a NPE when the optional flood rate parameter is not set by adding
a check before the value is used

Change-Id: I73a028199bf49a08be5df6f9a72b4cb12fbd1343
Signed-off-by: Eli Mesika <emesika@redhat.com>
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=908775