Fix for handling of empty lists of eventmessages

DocumentPerCommit strategy throws exception when dealing with empty
lists of event messages.

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$CommitEntry.<init>(DocumentPerCommitStorageStrategy.java:178)
    at org.axonframework.eventstore.mongo.DocumentPerCommitStorageStrategy$CommitEntry.<init>(DocumentPerCommitStorageStrategy.java:144)
    at org.axonframework.eventstore.mongo.DocumentPerCommitStorageStrategy.createDocuments(DocumentPerCommitStorageStrategy.java:89)
Moved empty check for event stream to MongoEventStore

The event store shouldn't call the CommitStrategy if the stream of events to
commit is empty.

Issue #AXON-176 Fixed