Don't serialise empty mod settings

osu.Game.Tests/Online/TestAPIModJsonSerialization.cs

@@ -6,6 +6,7 @@
 using System;
 using System.Collections.Generic;
 using Newtonsoft.Json;
+using Newtonsoft.Json.Linq;
 using NUnit.Framework;
 using osu.Framework.Bindables;
 using osu.Game.Beatmaps;
@@ -148,6 +149,16 @@ public void TestAPIModDetachedFromSource()
             Assert.That(apiMod.Settings["speed_change"], Is.EqualTo(1.01d));
         }
 
+        [Test]
+        public void TestSerialisedModSettingPresence()
+        {
+            var mod = new TestMod();
+
+            mod.TestSetting.Value = mod.TestSetting.Default;
+            JObject serialised = JObject.Parse(JsonConvert.SerializeObject(new APIMod(mod)));
+            Assert.False(serialised.ContainsKey("settings"));
+        }
+
         private class TestRuleset : Ruleset
         {
             public override IEnumerable<Mod> GetModsFor(ModType type) => new Mod[]

osu.Game/Online/API/APIMod.cs

@@ -1,11 +1,8 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
-#nullable disable
-
 using System;
 using System.Collections.Generic;
-using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using MessagePack;
 using Newtonsoft.Json;
@@ -23,7 +20,7 @@ public class APIMod : IEquatable<APIMod>
     {
         [JsonProperty("acronym")]
         [Key(0)]
-        public string Acronym { get; set; }
+        public string Acronym { get; set; } = string.Empty;
 
         [JsonProperty("settings")]
         [Key(1)]
@@ -49,7 +46,7 @@ public APIMod(Mod mod)
             }
         }
 
-        public Mod ToMod([NotNull] Ruleset ruleset)
+        public Mod ToMod(Ruleset ruleset)
         {
             Mod resultMod = ruleset.CreateModFromAcronym(Acronym);
 
@@ -80,6 +77,8 @@ public Mod ToMod([NotNull] Ruleset ruleset)
             return resultMod;
         }
 
+        public bool ShouldSerializeSettings() => Settings.Count > 0;
+
         public bool Equals(APIMod other)
         {
             if (ReferenceEquals(null, other)) return false;