FIX: missing reaction user for heart emoji breaks post mover

[dbattersby]

When a user reacts to a post it creates an entry for both the discourse_reactions_reactions and discourse_reactions_reaction_users tables.

However, when a post is liked using the ❤️ emoji it still creates a discourse_reactions_reactions entry but stores user data within post_actions in core and doesn't create a discourse_reactions_reaction_users entry.

This change adds an extra check to skip adding DiscourseReactions::ReactionUser data if none is found.

The new test recreates the logic outlined above to assume that the post has a reaction and not the reaction user when it is liked, and should still work correctly.

[jjaffeux]

LGTM