Handle xx as TMDb no language for backdrops (#14941)

MediaBrowser.Providers/Plugins/Tmdb/TmdbUtils.cs

@@ -185,7 +185,10 @@ namespace MediaBrowser.Providers.Plugins.Tmdb
                 return requestLanguage;
             }
 
-            return imageLanguage;
+            // TMDb now returns xx for no language instead of an empty string.
+            return string.Equals(imageLanguage, "xx", StringComparison.OrdinalIgnoreCase)
+                ? string.Empty
+                : imageLanguage;
         }
 
         /// <summary>

tests/Jellyfin.Providers.Tests/Tmdb/TmdbUtilsTests.cs

@@ -29,6 +29,7 @@ namespace Jellyfin.Providers.Tests.Tmdb
         [InlineData("fr-CA", "fr-BE", "fr-CA")]
         [InlineData("fr-CA", "fr", "fr-CA")]
         [InlineData("de", "en-US", "de")]
+        [InlineData("", "en-US", "")]
         public static void AdjustImageLanguage_Valid_Success(string imageLanguage, string requestLanguage, string? expected)
         {
             Assert.Equal(expected, TmdbUtils.AdjustImageLanguage(imageLanguage, requestLanguage));