feat: have sql_get_actor return null when no actor is found

public/api/v1/database.php

@@ -19,5 +19,8 @@ function sql_fetch_actor($conn, string $preferred_username) {
 		array(':object_id' => $object_id)
 	);
 
-	return $result->fetchArray(SQLITE3_ASSOC);
+	// fetchArray returns false when there's no array; we're rather have it be
+	// explicitly null
+	$actor = $result->fetchArray(SQLITE3_ASSOC);
+	return $actor !== false ? $actor : null;
 }