minor fix in CBLDocument after ChangeListener implementation

it fixes NullPointerException when we iterate
'for (ChangeListener listener : changeListeners)' in revisionAdded

failed tests:
com.couchbase.cblite.DocumentTest > testDeleteDocument[MyAvd0(AVD) - 4.2.2] FAILED
	java.lang.NullPointerException
	at com.couchbase.cblite.CBLDocument.revisionAdded(CBLDocument.java:431)

com.couchbase.cblite.DocumentTest > testNewDocumentHasCurrentRevision[MyAvd0(AVD) - 4.2.2] FAILED
	java.lang.NullPointerException
	at com.couchbase.cblite.CBLDocument.revisionAdded(CBLDocument.java:431)

com.couchbase.cblite.Views > testLargerViewQuery[MyAvd0(AVD) - 4.2.2] FAILED
	java.lang.NullPointerException
	at com.couchbase.cblite.CBLDocument.revisionAdded(CBLDocument.java:431)