api/graph/events: Fix panic for external participants in SendEmail

Previously, SQL queries in SendEmail would return NULL for participant
types other than 'user' or 'email'. This would panic for participant
types like 'external' since NULL cannot be scanned to a string, which
caused the submitTicket mutation to fail when called with an externalId.
Fix by returning and checking for an empty string instead.

api/graph/events.go

@@ -295,12 +295,12 @@ func (builder *EventBuilder) SendEmails(subject string,
 			CASE part.participant_type
 			WHEN 'user' THEN '~' || "user".username
 			WHEN 'email' THEN part.email_name
-			ELSE null END
+			ELSE '' END
 		`, `
 			CASE part.participant_type
 			WHEN 'user' THEN "user".email
 			WHEN 'email' THEN part.email
-			ELSE null END
+			ELSE '' END
 		`).
 		Distinct().
 		From(`event_participant evpart`).
@@ -320,6 +320,9 @@ func (builder *EventBuilder) SendEmails(subject string,
 		if err := rows.Scan(&name, &address); err != nil {
 			panic(err)
 		}
+		if len(name) == 0 || len(address) == 0 {
+			continue
+		}
 		if address == user.Email {
 			if notifySelf {
 				copiedSelf = true