Polishing.

Reformat code, revise nullability declarations.

See #4709
Original pull request: #4721

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/repository/support/SpringDataMongodbSerializer.java

@@ -115,7 +115,7 @@ class SpringDataMongodbSerializer extends MongodbDocumentSerializer {
 		return asReference(constant, null);
 	}
 
-	protected DBRef asReference(Object constant, Path<?> path) {
+	protected DBRef asReference(Object constant, @Nullable Path<?> path) {
 		return converter.toDBRef(constant, getPropertyForPotentialDbRef(path));
 	}
 
@@ -135,7 +135,7 @@ class SpringDataMongodbSerializer extends MongodbDocumentSerializer {
 
 		MongoPersistentProperty property = getPropertyFor(path);
 
-		return property.isIdProperty() ? key.replaceAll("." + ID_KEY + "$", "") : key;
+		return property != null && property.isIdProperty() ? key.replaceAll("." + ID_KEY + "$", "") : key;
 	}
 
 	@Override
@@ -144,8 +144,14 @@ class SpringDataMongodbSerializer extends MongodbDocumentSerializer {
 		return propertyFor == null ? super.isId(arg) : propertyFor.isIdProperty();
 	}
 
+	@Override
+	@Nullable
 	protected Object convert(@Nullable Path<?> path, @Nullable Constant<?> constant) {
 
+		if (constant == null) {
+			return null;
+		}
+
 		if (!isReference(path)) {
 
 			MongoPersistentProperty property = getPropertyFor(path);
@@ -166,6 +172,7 @@ class SpringDataMongodbSerializer extends MongodbDocumentSerializer {
 
 		MongoPersistentProperty property = getPropertyFor(path);
 
+		if (property != null) {
 			if (property.isDocumentReference()) {
 				return converter.toDocumentPointer(constant.getConstant(), property).getPointer();
 			}
@@ -178,6 +185,7 @@ class SpringDataMongodbSerializer extends MongodbDocumentSerializer {
 				}
 				return asReference(constant.getConstant(), path.getMetadata().getParent());
 			}
+		}
 
 		return asReference(constant.getConstant(), path);
 	}
@@ -203,7 +211,8 @@ class SpringDataMongodbSerializer extends MongodbDocumentSerializer {
 	 * @param path
 	 * @return
 	 */
-	private MongoPersistentProperty getPropertyForPotentialDbRef(Path<?> path) {
+	@Nullable
+	private MongoPersistentProperty getPropertyForPotentialDbRef(@Nullable Path<?> path) {
 
 		if (path == null) {
 			return null;