Calling .empty? for nil from aasm_ensure_initial_state with Mongoid

[zedtux]

Describe the bug
Since the commit 1c5117, and starting with AASM version 5.0.5, AASM is no more compatible with Mongoid aliased fields.

You can find a code example from #535 which makes AASM breaking with :

      NoMethodError:
        undefined method `empty?' for nil:NilClass
      # /usr/local/bundle/gems/aasm-5.0.5/lib/aasm/persistence/mongoid_persistence.rb:109:in `block in aasm_ensure_initial_state'
      # /usr/local/bundle/gems/aasm-5.0.5/lib/aasm/persistence/mongoid_persistence.rb:103:in `each'
      # /usr/local/bundle/gems/aasm-5.0.5/lib/aasm/persistence/mongoid_persistence.rb:103:in `aasm_ensure_initial_state'
      # /usr/local/bundle/gems/mongoid-7.5.1/lib/mongoid/interceptable.rb:122:in `run_callbacks'
      # /usr/local/bundle/gems/mongoid-7.5.1/lib/mongoid/document.rb:116:in `block in initialize'
      # ...

To Reproduce
Steps to reproduce the behavior:

1. Create a model using a Mongoid aliased field like in Mongoid initial value not setted (new case or regression) #535
2. Try to call .new on it
3. See error

Expected behavior
The model initialization should work without any errors.

[bk-one]

+1