Fix NPE when traversing map.

We now use regular iteration instead of the Stream API.

Closes: #4567
Original pull request: #4568

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

@@ -607,11 +607,11 @@ public class QueryMapper {
 			return source;
 		}
 
-		if (source instanceof Map) {
+		if (source instanceof Map<?,?> sourceMap) {
 
-			Map<String, Object> map = new LinkedHashMap<>();
+			Map<String, Object> map = new LinkedHashMap<>(sourceMap.size(), 1F);
 
-			((Map<String, Object>) source).entrySet().forEach(it -> {
+			sourceMap.entrySet().forEach(it -> {
 
 				String key = ObjectUtils.nullSafeToString(converter.convertToMongoType(it.getKey()));
 

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

@@ -768,6 +768,18 @@ class UpdateMapperUnitTests {
 		assertThat(mappedUpdate).doesNotContainKey("$set.concreteMap.jasnah._class");
 	}
 
+	@Test // GH-4567
+	void updateShouldAllowNullValuesInMap() {
+
+		Map<Object, NestedDocument> map = Collections.singletonMap("jasnah", new NestedDocument("kholin"));
+
+		Update update = new Update().set("concreteMap", Collections.singletonMap("jasnah", null));
+		Document mappedUpdate = mapper.getMappedObject(update.getUpdateObject(),
+				context.getPersistentEntity(EntityWithObjectMap.class));
+
+		assertThat(mappedUpdate).isEqualTo(new Document("$set", new Document("concreteMap", Collections.singletonMap("jasnah", null))));
+	}
+
 	@Test // DATAMONGO-1250
 	@SuppressWarnings("unchecked")
 	void mapsUpdateWithBothReadingAndWritingConverterRegistered() {