Description of problem: when agent does not find any of file requested by server to deliver its content, it fails, we need to add check for such situation (which can occur when all files are deleted at a time between agent reported file paths & SHAs and server processed it and requests content of some files)


Version-Release number of selected component (if applicable):
Version: 3.0.0.GA
Build Number: e23441b:4e746e8

How reproducible:always
  
Actual results:

Agent shows up exception

ERROR [pool-3-thread-1] (rhq.core.pc.drift.DriftManager)- An error occurred while trying to send content for changeset[resourceId: 10004, driftDefinition: Template-File System]
java.util.zip.ZipException: ZIP file must have at least one entry
	at java.util.zip.ZipOutputStream.finish(ZipOutputStream.java:304)
	at java.util.zip.DeflaterOutputStream.close(DeflaterOutputStream.java:146)
	at java.util.zip.ZipOutputStream.close(ZipOutputStream.java:321)
	at org.rhq.core.pc.drift.DriftManager.sendChangeSetContentToServer(DriftManager.java:362)
	at org.rhq.core.pc.drift.DriftFilesSender.run(DriftFilesSender.java:70)
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:441)
	at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
	at java.util.concurrent.FutureTask.run(FutureTask.java:138)
	at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(ScheduledThreadPoolExecutor.java:98)
	at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:206)
	at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
	at java.lang.Thread.run(Thread.java:662)


Expected results: No such exception occurs