Small fixes

Author: papafe

src/MongoDB.Driver/Core/WireProtocol/Messages/Encoders/JsonEncoders/QueryMessageJsonEncoder.cs

@@ -31,7 +31,8 @@ internal sealed class QueryMessageJsonEncoder : MessageJsonEncoderBase, IMessage
         public QueryMessageJsonEncoder(TextReader textReader, TextWriter textWriter, MessageEncoderSettings encoderSettings)
             : base(textReader, textWriter, encoderSettings)
         {
-            _serializationDomain = encoderSettings.GetOrDefault<IBsonSerializationDomain>(MessageEncoderSettingsName.SerializationDomain, null);
+            _serializationDomain = encoderSettings?.GetOrDefault<IBsonSerializationDomain>(MessageEncoderSettingsName.SerializationDomain, null) ?? BsonSerializer.DefaultSerializationDomain;
+            //QUESTION: Should we use the default serialization domain here? I think it's appropriate.
         }
 
         // methods

src/MongoDB.Driver/MongoClientSettings.cs

@@ -476,23 +476,18 @@ public ReadPreference ReadPreference
 
         IBsonSerializationDomain IInheritableMongoClientSettings.SerializationDomain
         {
-            get => _serializationDomain;
+            //QUESTION Is this reasonable?
+            get => _serializationDomain ?? BsonSerializer.DefaultSerializationDomain;
             set
             {
                 if (_isFrozen) { throw new InvalidOperationException("MongoClientSettings is frozen."); }
                 _serializationDomain = value ?? throw new ArgumentNullException(nameof(value));
             }
         }
 
+        //FP This is a convenience property, it could be removed.
         internal IBsonSerializationDomain SerializationDomain
-        {
-            get => _serializationDomain;
-            set
-            {
-                if (_isFrozen) { throw new InvalidOperationException("MongoClientSettings is frozen."); }
-                _serializationDomain = value ?? throw new ArgumentNullException(nameof(value));
-            }
-        }
+            => (this as IInheritableMongoClientSettings).SerializationDomain;
 
         /// <summary>
         /// Gets or sets the name of the replica set.

src/MongoDB.Driver/MongoCollectionSettings.cs

@@ -124,8 +124,7 @@ public ReadPreference ReadPreference
         /// </summary>
         internal IBsonSerializationDomain SerializationDomain
         {
-            //QUESTION Is this correct? Normally the domain would be setup by ApplyDefaultValues, but for testing it would not work.
-            //Internally we can also modify the tests to set this (when we use mocks), but externally developers won't be able to set this.
+            //QUESTION Is this reasonable? In normal uses cases the domain would be setup by ApplyDefaultValues, but for testing it would not work.
             get => _serializationDomain.Value ?? BsonSerializer.DefaultSerializationDomain;
             set
             {

src/MongoDB.Driver/MongoDatabaseSettings.cs

@@ -109,7 +109,7 @@ public ReadPreference ReadPreference
         /// </summary>
         internal IBsonSerializationDomain SerializationDomain
         {
-            get => _serializationDomain.Value;
+            get => _serializationDomain.Value ?? BsonSerializer.DefaultSerializationDomain; //QUESTION Is this reasonable?
             set
             {
                 if (_isFrozen) { throw new InvalidOperationException("MongoCollectionSettings is frozen."); }