Ensure that MongoClient is not created with a null credentials list

Mongo’s 2.x driver allowed the credentials list to be null, however
the 3.x driver requires an empty list instead. The behaviour of the 2.x
driver is the same whether the client is created with a null credential
list or an empty credential list.

This commit aligns with the requirements of the 3.x driver by ensuring
that we never pass in a null credential list when creating the client.

Closes gh-4076

Author: wilkinsona

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/mongo/MongoProperties.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2015 the original author or authors.
+ * Copyright 2012-2016 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -17,6 +17,7 @@
 package org.springframework.boot.autoconfigure.mongo;
 
 import java.net.UnknownHostException;
+import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
 
@@ -213,12 +214,12 @@ public MongoClient createMongoClient(MongoClientOptions options,
 				if (options == null) {
 					options = MongoClientOptions.builder().build();
 				}
-				List<MongoCredential> credentials = null;
+				List<MongoCredential> credentials = new ArrayList<MongoCredential>();
 				if (hasCustomCredentials()) {
 					String database = this.authenticationDatabase == null
 							? getMongoClientDatabase() : this.authenticationDatabase;
-					credentials = Arrays.asList(MongoCredential
-							.createCredential(this.username, database, this.password));
+					credentials.add(MongoCredential.createCredential(this.username,
+							database, this.password));
 				}
 				String host = this.host == null ? "localhost" : this.host;
 				int port = determinePort(environment);