Update AbstractTrigger.cs

Fix so null comparison no longer results in null ref exception
Merge pull request #176 from annakata/patch-1

Update AbstractTrigger.cs
fixes #183 cannot register trigger persistence delegates with assembly qualified names
2.2.4 release preparations
fixes #184 add FirebirdDelegate, Firebird DDL script, update Firebird driver metadata
fixes #188 CronExpression ctor incorrectly uses the non-uppercased string
Add the ability to check whether a calendar exists from the job store.
Update method name and formatting.
Merge pull request #192 from SeanFarrow/master

Add the ability to check whether a calendar exists from the job store.
fixes #193 Paused triggers are reported as blocked