Fix NPE when rendering untyped aggregation operation as part of criteria query.

Resolves: spring-projects#4687
Original pull request: spring-projects#4695

Author: christophstrobl

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/QueryMapper.java

@@ -548,8 +548,11 @@ protected Object convertSimpleOrDocument(Object source, @Nullable MongoPersisten
 		}
 
 		if (source instanceof AggregationExpression age) {
-			return age
-					.toDocument(new RelaxedTypeBasedAggregationOperationContext(entity.getType(), this.mappingContext, this));
+
+			if(entity == null) {
+				return age.toDocument();
+			}
+			return age.toDocument(new RelaxedTypeBasedAggregationOperationContext(entity.getType(), this.mappingContext, this));
 		}
 
 		if (source instanceof MongoExpression exr) {

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/convert/QueryMapperUnitTests.java

@@ -1604,6 +1604,15 @@ void usageOfMongoExpressionOnCriteriaDoesNotUnwrapAnExprAggregationExpression()
 		assertThat(mappedObject).isEqualTo("{ $expr : { $expr : { $gt : [ '$foo', '$budget'] } } }");
 	}
 
+	@Test // GH-4687
+	void usageOfUntypedAggregationShouldRenderOperationsAsIs() {
+
+		Query query = query(expr(Expr.valueOf(ComparisonOperators.valueOf("field").greaterThan("budget"))));
+		org.bson.Document mappedObject = mapper.getMappedObject(query.getQueryObject(),
+			context.getPersistentEntity(Object.class));
+		assertThat(mappedObject).isEqualTo("{ $expr : { $expr : { $gt : [ '$field', '$budget'] } } }");
+	}
+
 	@Test // GH-2750
 	void usesMongoExpressionDocumentAsIsIfItIsNotAnAggregationExpression() {