If a field is nil for a specific locale, Entry gets into a corrupted state

[dlitvakb]

To fix, make sure that Entry#fields always at least returns an empty hash.

[32] pry(main)> client.entries('sys.id' => 'no-ne-NP', locale: 'ne-NP')
=> #<Contentful::Array:0x3ff3558c2610>
[33] pry(main)> client.entries('sys.id' => 'no-ne-NP', locale: 'ne-NP').first
=> #<Contentful::DynamicEntry[sampleType]:0x3ff35729dabc>
[34] pry(main)> client.entries('sys.id' => 'no-ne-NP', locale: 'ne-NP').first.fields
=> nil
[35] pry(main)> client.entries('sys.id' => 'no-ne-NP', locale: 'ne-NP').first.instance_variable_get(:@fields)
=> {}
[36] pry(main)> client.entries('sys.id' => 'no-ne-NP', locale: 'ne-NP').first.title
NoMethodError: undefined method `[]' for nil:NilClass
from /Users/davemac/dev/contentful/api_clients/ruby/delivery.rb/lib/contentful/dynamic_entry.rb:36:in `block (3 levels) in create'
[37] pry(main)> client.entries('sys.id' => 'no-ne-NP', locale: 'ne-NP').first.respond_to?(:title)
=> true