json_include(nil) returns the base document and may cause false-passing tests

[Startouf]

I was trying to assert an included jsonapi document with

# Assume `class Foo has_one(:bar, polymorphic: true)`
describe 'foos#show' do
  context 'with included bar' do 
    foo_in_db, bar_in_db = seed_a_foo_with_a_bar # Foo.find(42).bar exists
    get "/foos/42", params: { include: 'bar }
    foo_json = json_item
    bar_included_in_foo_json = json_include(json_included_types.first) 
    expect(json_included_item).to be
    assert_payload(:'bar_payload', bar_in_db, bar_included_in_foo_json)

The problem : there was a bug in my application code (irrelevant here) and json_included_types.first was nil/didn't contain my expected Bar, therefore resulting in a json_include(nil) which did not return an error and instead returned a version of the main json_item (Foo) stripped of its metadata, causing the test to pass when it actually failed