DATAMONGO-2616 - Polishing.

Reformat code. Merge if-statements.

Original pull request: #889.

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

@@ -127,11 +127,7 @@ public interface MongoConverter
 	@Nullable
 	default Object convertId(@Nullable Object id, Class<?> targetType) {
 
-		if (id == null) {
-			return null;
-		}
-
-		if(ClassUtils.isAssignableValue(targetType, id)) {
+		if (id == null || ClassUtils.isAssignableValue(targetType, id)) {
 			return id;
 		}