fix condition for filter gambit (search only)

[novacuum]

Fixes #0000

Report by @novacuum

I stumbled upon this change: https://github.com/FriendsOfFlarum/best-answer/commit/b97010304707e1be311d65385a88f17ffc16acad
My indention was to select discussions without a solved post, but this is not possible, as in the negation it excludes the discussions which have the tags with qna enabled.
From my understanding, the method call in line 54 (old code) was correct, only the field was wrong.
It should only change the check for discussions.best_answer_post_id , is null or notnull

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Confirmed

Frontend changes: tested on a local Flarum installation.

Backend changes: tests are green (run composer test).

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Related Flarum core extension PR's: (Omit this section if irrelevant)