CSHARP-1622: Fixed minor issue that was causing some tests to fail.

Author: rstam

src/MongoDB.Driver/IAggregateFluentExtensions.cs

@@ -506,7 +506,7 @@ private sealed class ProjectExpressionProjection<TResult, TNewResult> : Projecti
             public ProjectExpressionProjection(Expression<Func<TResult, TNewResult>> expression, ExpressionTranslationOptions translationOptions)
             {
                 _expression = Ensure.IsNotNull(expression, nameof(expression));
-                _translationOptions = translationOptions;
+                _translationOptions = translationOptions; // can be null
             }
 
             public Expression<Func<TResult, TNewResult>> Expression
@@ -530,7 +530,7 @@ public GroupExpressionProjection(Expression<Func<TResult, TKey>> idExpression, E
             {
                 _idExpression = Ensure.IsNotNull(idExpression, nameof(idExpression));
                 _groupExpression = Ensure.IsNotNull(groupExpression, nameof(groupExpression));
-                _translationOptions = translationOptions;
+                _translationOptions = translationOptions; // can be null
             }
 
             public Expression<Func<TResult, TKey>> IdExpression

src/MongoDB.Driver/Linq/Translators/QueryableTranslator.cs

@@ -58,7 +58,7 @@ public static QueryableTranslation Translate(Expression node, IBsonSerializerReg
         public QueryableTranslator(IBsonSerializerRegistry serializerRegistry, ExpressionTranslationOptions translationOptions)
         {
             _serializerRegistry = Ensure.IsNotNull(serializerRegistry, nameof(serializerRegistry));
-            _translationOptions = Ensure.IsNotNull(translationOptions, nameof(translationOptions));
+            _translationOptions = translationOptions; // can be null
             _stages = new List<BsonDocument>();
         }