fix(meta): store unrated titles as NULL, not zero

TMDB reports vote_average: 0 where no rating exists (#156). Normalise
it to None at the arr-meta edge, like non_empty does for "", and let
the Option flow through the daemon refresh and API add paths so the
nullable columns from #145 do their job.

crates/arr-meta/tests/tmdb.rs

@@ -217,6 +217,9 @@ async fn unreleased_movie_has_no_dates_and_no_imdb_id() {
     assert_eq!(movie.imdb_id, None);
     assert_eq!(movie.runtime, None);
     assert!(movie.origin_countries.is_empty());
+    // TMDB reports vote_average: 0 for an unrated title — absence, not zero
+    // (#156), so it is normalised away like any other zero-valued null.
+    assert_eq!(movie.vote_average, None);
     assert!(!movie.is_digitally_released(date(2026, 8, 22)));
 }