New: Fallback to tmdb search on Rarbg if IMDb is null

src/NzbDrone.Core/Indexers/Rarbg/RarbgRequestGenerator.cs

@@ -87,10 +87,14 @@ namespace NzbDrone.Core.Indexers.Rarbg
 
             requestBuilder.AddQueryParam("mode", "search");
 
-            if (searchCriteria.Movie.ImdbId != null)
+            if (searchCriteria.Movie.ImdbId.IsNotNullOrWhiteSpace())
             {
                 requestBuilder.AddQueryParam("search_imdb", searchCriteria.Movie.ImdbId);
             }
+            else if (searchCriteria.Movie.TmdbId > 0)
+            {
+                requestBuilder.AddQueryParam("search_themoviedb", searchCriteria.Movie.TmdbId);
+            }
             else
             {
                 requestBuilder.AddQueryParam("search_string", $"{searchCriteria.Movie.Title} {searchCriteria.Movie.Year}");