feat(alerting): make post execution fields nullable

pkg/services/sqlstore/migrations/alert_mig.go

@@ -22,9 +22,9 @@ func addAlertMigrations(mg *Migrator) {
 			{Name: "severity", Type: DB_Text, Nullable: false},
 			{Name: "paused", Type: DB_Bool, Nullable: false},
 			{Name: "silenced", Type: DB_Bool, Nullable: false},
-			{Name: "execution_error", Type: DB_Text, Nullable: false},
-			{Name: "last_eval_data", Type: DB_Text, Nullable: false},
-			{Name: "last_eval_time", Type: DB_DateTime, Nullable: false},
+			{Name: "execution_error", Type: DB_Text, Nullable: true},
+			{Name: "last_eval_data", Type: DB_Text, Nullable: true},
+			{Name: "last_eval_time", Type: DB_DateTime, Nullable: true},
 			{Name: "created", Type: DB_DateTime, Nullable: false},
 			{Name: "updated", Type: DB_DateTime, Nullable: false},
 			{Name: "updated_by", Type: DB_BigInt, Nullable: false},