[eclipselink-dev] fix for bug 328293 checked in

[Christopher Delahunt <christopher.delahunt@xxxxxxxxxx>]

https://bugs.eclipse.org/bugs/show_bug.cgi?id=328293\

328293 - testUpdateWithProxy() failed with NPE in J2SE and J2EE in 
EclipseLink trunk
-AbstractSession's writesCompleted now checks if the value from 
getAccessor() is null before calling writesCompleted on it.  
This would be null only when an ExclusiveIsolatedClientSession is used, 
and flush is called without any changes being made.  

Checked into 2.2, revision 8382
Reviewed by: Andrei Ilitchev