Mongo docpercommit strategy

DocumentPerCommit strategy throws exception when dealing with empty lists of event messages.

```java 
   java.lang.IndexOutOfBoundsException: Index: 0, Size: 0
        at java.util.ArrayList.RangeCheck(ArrayList.java:547)
        at java.util.ArrayList.get(ArrayList.java:322)
        at org.axonframework.eventstore.mongo.DocumentPerCommitStorageStrategy$CommitEntry.<init>(DocumentPerCommitStorage
        at org.axonframework.eventstore.mongo.DocumentPerCommitStorageStrategy$CommitEntry.<init>(DocumentPerCommitStorage
        at org.axonframework.eventstore.mongo.DocumentPerCommitStorageStrategy.createDocuments(DocumentPerCommitStorageStr
```