Add prototype serialization tests. (#18458)

Co-authored-by: metalgearsloth <31366439+metalgearsloth@users.noreply.github.com>

Content.Server/Holiday/Celebrate/DefaultHolidayCelebrate.cs

@@ -1,12 +0,0 @@
-using Content.Server.Holiday.Interfaces;
-
-namespace Content.Server.Holiday.Celebrate
-{
-    public sealed class DefaultHolidayCelebrate : IHolidayCelebrate
-    {
-        public void Celebrate(HolidayPrototype holiday)
-        {
-            // Nada.
-        }
-    }
-}

Content.Server/Holiday/Greet/DefaultHolidayGreet.cs

@@ -2,6 +2,7 @@ using Content.Server.Holiday.Interfaces;
 
 namespace Content.Server.Holiday.Greet
 {
+    [DataDefinition]
     public sealed class DefaultHolidayGreet : IHolidayGreet
     {
         public string Greet(HolidayPrototype holiday)

Content.Server/Holiday/HolidayPrototype.cs

@@ -1,4 +1,3 @@
-using Content.Server.Holiday.Celebrate;
 using Content.Server.Holiday.Greet;
 using Content.Server.Holiday.Interfaces;
 using Content.Server.Holiday.ShouldCelebrate;
@@ -40,7 +39,7 @@ namespace Content.Server.Holiday
         private readonly IHolidayGreet _greet = new DefaultHolidayGreet();
 
         [DataField("celebrate")]
-        private readonly IHolidayCelebrate _celebrate = new DefaultHolidayCelebrate();
+        private readonly IHolidayCelebrate? _celebrate = null;
 
         public bool ShouldCelebrate(DateTime date)
         {
@@ -57,7 +56,7 @@ namespace Content.Server.Holiday
         /// </summary>
         public void Celebrate()
         {
-            _celebrate.Celebrate(this);
+            _celebrate?.Celebrate(this);
         }
     }
 }

Content.Server/Holiday/ShouldCelebrate/DefaultHolidayShouldCelebrate.cs

@@ -2,7 +2,7 @@ using Content.Server.Holiday.Interfaces;
 
 namespace Content.Server.Holiday.ShouldCelebrate
 {
-    [Virtual]
+    [Virtual, DataDefinition]
     public class DefaultHolidayShouldCelebrate : IHolidayShouldCelebrate
     {
         public virtual bool ShouldCelebrate(DateTime date, HolidayPrototype holiday)