FIX CI: Parent_id should equals to nil - it is default value in request_specs

[lpichler]

caused by removing this https://github.com/ManageIQ/manageiq/pull/18301/files#diff-5bb77b717064efbd95e6ab04eb5a357fL5 (in ManageIQ/manageiq#18301)

so I update condition to match nil.

cc @kbrock @d-m-u
@miq-bot assign @jrafanie

[miq-bot]

Checked commit lpichler@9a6b74e with ruby 2.3.3, rubocop 0.69.0, haml-lint 0.20.0, and yamllint 1.10.0
1 file checked, 0 offenses detected
Everything looks fine. 🍪

[kbrock]

after note:

In the future, lets use is_category for stuff like this:

Classification.is_category.where(:description => 'Department').includes(:tag).first

I'm surprised there were any :parent_id => *** left anywhere in our code