Fix error when user()->first() is null in DiscussionBestAnswerPage.php

[alexisrazok]

Fix to avoid error message when $discussion->user()->first() is null.

[jaspervriends]

Hey @alexisrazok , thanks for the PR! I see Clark just replied with a different kind of solution as well in #101 (he doesn't have a PR), would you be able to check if that works and change your PR? Then I'll go ahead and merge the PR for you 😄

#101

[alexisrazok]

yes @jaspervriends it works, I made a mistake in the other patch but this patch is right. Thank you.

[rafaucau]

Since this hasn't been merged yet, I have opened a new pull request that includes several fixes, including this one:
#102

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By the way, this can be simplified (PHP 8.0+) and optimized to avoid unnecessary database queries, because user is already loaded:

'author' => [
    "@type" => "Person",
    "name" => $discussion->user?->getDisplayNameAttribute()
],