A serializer without relationships breaks when passing include option to .new #210

Expected Behavior

A validation error (JSONAPI::Serializer::UnsupportedIncludeError) should be raised instead of breaking.

Actual Behavior

A NoMethodError is raised when trying to access .relationships_to_serialize, which is nil.

Steps to Reproduce the Problem

class S; include JSONAPI::Serializer; set_type :s; end
S.new({}, include: %i[invalid])
# => NoMethodError: undefined method `[]' for nil:NilClass from .../gems/jsonapi-serializer-2.2.0/lib/fast_jsonapi/object_serializer.rb:344:in `block in validate_includes!'

Specifications

• Version: 2.2.0
• Ruby version: 2.7.5

Discussion

Related, but possibly off-topic:

I also do a little bit of reflection on serializers, and the behavior of attributes_to_serialize and relationships_to_serialize - returning nil when empty - is kinda annoying. It's easy to workaround by appending .to_h to the returned values, but in this case I don't control how the gem itself handles it.

I saw that throughout the gem's code there are checks on these values. Wouldn't be easier to set default values on them instead? Likely here:

included do
  class << self
    attr_accessor :attributes_to_serialize,
      :relationships_to_serialize #, ...

    self.attributes_to_serialize = {}
    self.relationships_to_serialize = {}
  end
end

Or to use class_attribute, from active_support, which would handle inheritance and replace the manual handling?

included do
  class_attribute :attributes_to_serialize, default: {}
  class_attribute :relationships_to_serialize, default: {}
  # ...
end

(It would require taking note of the caveat in class_attribute's documentation: when changing mutable objects, setters must be used instead of directly modifying the object.)