Bugfix: Do not interpolate empty array into SQL

This fixes an edge case, a taxon condition without any taxons defined.
Prior to this commit, that would error on MySQL and Postgres.

Author: mamhoff

promotions/app/models/concerns/solidus_promotions/conditions/taxon_condition.rb

@@ -52,6 +52,8 @@ def taxon_ids_with_children
       end
 
       def load_taxon_ids_with_children
+        return [] unless taxon_ids.present?
+
         aggregation_function = if ActiveRecord::Base.connection.adapter_name.downcase.match?(/postgres/)
           "string_agg(child.id::text, ',')"
         else