use_camel_case: true causes nil reference error when accessing empty fields

[boxbinary]

The following line references content_type but if you have created the client with use_camel_case: true, the keys of sys have been instantiated using camelCase (e.g. it should be contentType in this case), and a nil reference error is thrown trying to access sys[:content_type].id

contentful.rb/lib/contentful/entry.rb

Line 108 in 487d044

sys[:content_type].id

This makes it impossible to use camel casing while also referencing fields that might be empty.

[dlitvakb]

The fix has been released in version 2.13.1.

Cheers