Improve default path for Synology Download Station

Fixes: Missing default path for Download Station
Fixes: Error when getting destination path for Synology Download Station in health check
Closes #4562

src/NzbDrone.Core.Test/Download/DownloadClientTests/DownloadStationTests/TorrentDownloadStationFixture.cs

@@ -419,7 +419,7 @@ namespace NzbDrone.Core.Test.Download.DownloadClientTests.DownloadStationTests
             id.Should().NotBeNullOrEmpty();
 
             Mocker.GetMock<IDownloadStationTaskProxy>()
-                  .Verify(v => v.AddTaskFromUrl(It.IsAny<string>(), null, It.IsAny<DownloadStationSettings>()), Times.Once());
+                  .Verify(v => v.AddTaskFromUrl(It.IsAny<string>(), _defaultDestination, It.IsAny<DownloadStationSettings>()), Times.Once());
         }
 
         [Test]

src/NzbDrone.Core.Test/Download/DownloadClientTests/DownloadStationTests/UsenetDownloadStationFixture.cs

@@ -298,7 +298,7 @@ namespace NzbDrone.Core.Test.Download.DownloadClientTests.DownloadStationTests
             id.Should().NotBeNullOrEmpty();
 
             Mocker.GetMock<IDownloadStationTaskProxy>()
-                  .Verify(v => v.AddTaskFromData(It.IsAny<byte[]>(), It.IsAny<string>(), null, It.IsAny<DownloadStationSettings>()), Times.Once());
+                  .Verify(v => v.AddTaskFromData(It.IsAny<byte[]>(), It.IsAny<string>(), _defaultDestination, It.IsAny<DownloadStationSettings>()), Times.Once());
         }
 
         [Test]

src/NzbDrone.Core/Download/Clients/DownloadStation/TorrentDownloadStation.cs

@@ -441,14 +441,15 @@ namespace NzbDrone.Core.Download.Clients.DownloadStation
             {
                 return Settings.TvDirectory.TrimStart('/');
             }
-            else if (Settings.TvCategory.IsNotNullOrWhiteSpace())
-            {
-                var destDir = GetDefaultDir();
+ 
+            var destDir = GetDefaultDir();
 
+            if (Settings.TvCategory.IsNotNullOrWhiteSpace())
+            {
                 return $"{destDir.TrimEnd('/')}/{Settings.TvCategory}";
             }
 
-            return null;
+            return destDir.TrimEnd('/');
         }
     }
 }

src/NzbDrone.Core/Download/Clients/DownloadStation/UsenetDownloadStation.cs

@@ -418,14 +418,15 @@ namespace NzbDrone.Core.Download.Clients.DownloadStation
             {
                 return Settings.TvDirectory.TrimStart('/');
             }
-            else if (Settings.TvCategory.IsNotNullOrWhiteSpace())
-            {
-                var destDir = GetDefaultDir();
 
+            var destDir = GetDefaultDir();
+
+            if (Settings.TvCategory.IsNotNullOrWhiteSpace())
+            {
                 return $"{destDir.TrimEnd('/')}/{Settings.TvCategory}";
             }
 
-            return null;
+            return destDir.TrimEnd('/');
         }
     }
 }