[BUGFIX] Fixed EagerLoadingMixin early return error

[calumbell]

Description

This PR fixes a bug in the EagerLoadingMixin where the follow conditional:

if not requested_fields:

Would never evaluate as True due to requested_fields being an array and arrays being Truthy in Python. This was replaced with a call to any() (which outputs true if any of the elements of an array is not false/falsey)

if not any(requested_fields):

The way that the fields were filtered in this mixin was also refactored to fix some additoinal bugs that taking a look at this issue exposed: specifically fields being filtered by sub-string matches, which might mess with nested fields with the same name, ie. a Creatures name matching to its document__name.

Related Issue

Closes #897

How was this tested

• pytest (all tests passing)
• Spot checked on local Django develpoment server