CSHARP-3898: Translate string.IsNullOrEmpty using $in instead of $or/$eq

Author: rstam

src/MongoDB.Driver/Linq/Linq3Implementation/Translators/ExpressionToAggregationExpressionTranslators/MethodTranslators/IsNullOrEmptyMethodToAggregationExpressionTranslator.cs

@@ -16,7 +16,6 @@
 using System.Linq.Expressions;
 using MongoDB.Bson;
 using MongoDB.Bson.Serialization.Serializers;
-using MongoDB.Driver.Linq.Linq3Implementation.Ast;
 using MongoDB.Driver.Linq.Linq3Implementation.Ast.Expressions;
 using MongoDB.Driver.Linq.Linq3Implementation.Misc;
 
@@ -33,13 +32,7 @@ public static AggregationExpression Translate(TranslationContext context, Method
             {
                 var stringExpression = arguments[0];
                 var stringTranslation = ExpressionToAggregationExpressionTranslator.Translate(context, stringExpression);
-
-                var (stringVar, stringAst) = AstExpression.UseVarIfNotSimple("this", stringTranslation.Ast);
-                var ast = AstExpression.Let(
-                    stringVar,
-                    @in: AstExpression.Or(
-                        AstExpression.Eq(stringAst, BsonNull.Value),
-                        AstExpression.Eq(stringAst, "")));
+                var ast = AstExpression.In(stringTranslation.Ast, new BsonArray { BsonNull.Value, "" });
 
                 return new AggregationExpression(expression, ast, new BooleanSerializer());
             }

tests/MongoDB.Driver.Tests/Linq/Linq2ImplementationTestsOnLinq3/Translators/AggregateProjectTranslatorTests.cs

@@ -1549,7 +1549,7 @@ public void Should_translate_string_is_null_or_empty()
         {
             var result = Project(x => new { Result = string.IsNullOrEmpty(x.B) });
 
-            result.Projection.Should().Be("{ Result: { \"$or\": [{ $eq: [\"$B\", null] }, { $eq: [\"$B\", \"\"] } ] }, _id: 0 }");
+            result.Projection.Should().Be("{ Result: { $in : ['$B', [null, '']] }, _id: 0 }");
 
             result.Value.Result.Should().BeFalse();
         }