Fix exception when sending bulk event email
Fix exception when sending bulk event email

This fixes the case where sending event emails for groups with no group
admin would throw an exception.

To test:
1. Find an event for a group with no group admin (or set the group
admin to null)
2. Send a bulk event email
3. Email should send successfully

Connects #1739

maurizi/nyc-trees,azavea/nyc-trees,kdeloach/nyc-trees,kdeloach/nyc-trees,azavea/nyc-trees,maurizi/nyc-trees,maurizi/nyc-trees,azavea/nyc-trees,kdeloach/nyc-trees,azavea/nyc-trees,kdeloach/nyc-trees,maurizi/nyc-trees,azavea/nyc-trees,kdeloach/nyc-trees