Bail out of Flush if it’s already been called.

If there’s nothing for Flush to do, then just bail out. This makes sure
it’s safe to call Flush multiple times.

query.go

@@ -169,6 +169,9 @@ func (q *Query) PostgreSQLString() (string, error) {
 // `join`, then added onto the Query’s SQL string, with a space between the SQL string
 // and the expressions.
 func (q *Query) Flush(join string) *Query {
+	if len(q.expressions) < 1 {
+		return q
+	}
 	q.sql = strings.TrimSpace(q.sql) + " "
 	q.sql += strings.TrimSpace(strings.Join(q.expressions, join))
 	q.expressions = q.expressions[0:0]