STI normalizer fails when existing rule conditions are relation

[adzap]

See https://github.com/CanCanCommunity/cancancan/blob/959eee14e132b3f5e22b9e29649327eaf46efb37/lib/cancan/model_adapters/sti_normalizer.rb#L34

The merge presumes a hash of conditions but this raises error if the conditions are a relation.