Refactor properties to use 'required' instead of nullable with null default

Author: niteshsinghal85

test/OpenApiTests/ResourceInheritance/Models/Mansion.cs

@@ -8,7 +8,7 @@ namespace OpenApiTests.ResourceInheritance.Models;
 public sealed class Mansion : Residence
 {
     [Attr]
-    public string OwnerName { get; set; } = null!;
+    public required string OwnerName { get; set; }
 
     [HasMany]
     public ISet<StaffMember> Staff { get; set; } = new HashSet<StaffMember>();

test/OpenApiTests/ResourceInheritance/Models/Room.cs

@@ -14,5 +14,5 @@ public abstract class Room : Identifiable<long>
     public int? SurfaceInSquareMeters { get; set; }
 
     [HasOne]
-    public Residence Residence { get; set; } = null!;
+    public required Residence Residence { get; set; }
 }

test/OpenApiTests/RestrictedControllers/Channel.cs

@@ -17,7 +17,7 @@ public abstract class Channel : Identifiable<long>
     public bool? IsAdultOnly { get; set; }
 
     [HasOne]
-    public DataStream VideoStream { get; set; } = null!;
+    public required DataStream VideoStream { get; set; }
 
     [HasOne]
     public DataStream? UltraHighDefinitionVideoStream { get; set; }