Update inspect method to handle nil serializable_hash

[fcce]

serializable_hash may return nil

before

Entities::User.represent nil
# An error occurred when inspecting the object: #<NoMethodError: undefined method `map' for nil:NilClass>

after

Entities::User.represent nil
#<Entities::User:326200 nil>

[dblock]

Add a test and update CHANGELOG please?

[dblock]

@numbata or @fcce want to finish this one?

[dblock]

Merged this, thanks for your help @numbata!