Avoid default category on existing Transmission configurations

Co-authored-by: Mark McDowall <mark@mcdowall.ca>
(cherry picked from commit bd656ae7f66fc9224ef2a57857152ee5d54d54f8)

src/NzbDrone.Core/Download/Clients/Transmission/TransmissionSettings.cs

@@ -1,3 +1,4 @@
+using System.Text.Json.Serialization;
 using System.Text.RegularExpressions;
 using FluentValidation;
 using NzbDrone.Common.Extensions;
@@ -27,6 +28,7 @@ namespace NzbDrone.Core.Download.Clients.Transmission
     {
         private static readonly TransmissionSettingsValidator Validator = new ();
 
+        // This constructor is used when creating a new instance, such as the user adding a new Transmission client.
         public TransmissionSettings()
         {
             Host = "localhost";
@@ -35,6 +37,18 @@ namespace NzbDrone.Core.Download.Clients.Transmission
             MovieCategory = "radarr";
         }
 
+        // TODO: Remove this in v6
+        // This constructor is used when deserializing from JSON, it will set the
+        // category to the deserialized value, defaulting to null.
+        [JsonConstructor]
+        public TransmissionSettings(string movieCategory = null)
+        {
+            Host = "localhost";
+            Port = 9091;
+            UrlBase = "/transmission/";
+            MovieCategory = movieCategory;
+        }
+
         [FieldDefinition(0, Label = "Host", Type = FieldType.Textbox)]
         public string Host { get; set; }