AO3-6811 Fix tag nomination parented bug

[pmonfort]

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• Have you added tests for any changed functionality?
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Issue

https://otwarchive.atlassian.net/browse/AO3-6811

Purpose

When a canonical tag is saved, Tag#update_tag_nominations sets parented: true on matching TagNomination records.
However, it only checked for Fandom-type parents and only matched nominations with a blank parent_tagname (empty string).
This caused nominations to be incorrectly marked as parented: false when:

• The tag's parent was a non-Fandom type (e.g., a Media parent on a Fandom tag).
• The nomination had a nil parent_tagname instead of an empty string.

This PR fixes both issues by:

1. Checking all parent types instead of only Fandoms.
2. Including both "" and nil in the parent_names list so nominations with either blank value are matched.

Testing Instructions

1. Create a canonical fandom tag with a Media parent (but no Fandom parent).
2. Create a tag nomination for that fandom in a tag set.
3. Save the fandom tag.
4. Verify the nomination's parented field is true.

Credit

Pablo Monfort (he/him)

[sarken]

Assigning myself as reviewer since I saw some things I want to poke at, but I am beat so that's not gonna happen tonight