Add field/association level default_if option

[mcclayton]

Sometimes, you may want certain "empty" values to pass through to the default value other than nil.
This PR gives Blueprinter the ability to treat the following empty types as the default value (or nil if no default provided).

Blueprinter::EMPTY_COLLECTION

An empty array or empty active record collection.

Blueprinter::EMPTY_HASH

An empty hash.

Blueprinter::EMPTY_STRING

An empty string or symbol.

Field-level/Association-level Setting

class UserBlueprint < Blueprinter::Base
  identifier :uuid

  view :normal do
    # If first_name is an empty string, it will become "N/A"
    field :first_name, default_if: Blueprinter::EmptyString, default: "N/A"
    # If the projects association collection is empty, it will become nil
    association :projects, blueprint: ProjectBlueprint, default_if: Blueprinter::EmptyCollection
  end
end

[wai-procore]

wondering if this module name should be consistent with the file name empty_types.rb or vice versa?

[wai-procore]

one NB comment about the filepath vs module name discrepancy, otherwise lgtm

[philipqnguyen]

This is new feature. What do you think if we make it 0.21.0?

Also need update to CHANGELOG

[mcclayton]

Of course, great catch!

[philipqnguyen]

the vanity =D

[mcclayton]

Just following the template 😅