Make RightSupport::Data::HashTools#deep_merge! handle empty hash values right

Without my changes following hashes will not be merged correct(target will remain the same)
```ruby
{"TEST Explore Unset Inputs"=>{"parameters"=>{"SET_THEN_UNSET_INPUT"=>"hi"}}, "tags"=>[]}
{"TEST Explore Unset Inputs"=>{"parameters"=>{}}}
```