GuardDuty events with no severity generate bad notifications

[wes-novack]

When a GuardDuty Event occurs that does not have a severity, such as when you archive a finding, this guardduty-to-slack Lambda generates a junk notification to Slack, which looks like the below.

GuardDutyAPP  11:13
Finding in us-west-2 for Acct: XXXXXXXXXXXX
undefined
undefined
Severity
High
Region
us-west-2
Last Seen
<!date^NaN^{date} at {time} | undefined>

I've implemented a fix for this in my local copy & will submit a PR.