fixes bug in ondemand tablet hosting

[keith-turner]

When a hosting request was sent to the manager and the manager determined there was nothing to do it would attempt to create a batch scanner with an empty set of ranges and this would fail. Added a check to see if the ranges were empty.

[keith-turner]

Saw the following stack trace for this bug.

java.lang.IllegalArgumentException: ranges must be non null and contain at least 1 range
	at org.apache.accumulo.core.clientImpl.TabletServerBatchReader.setRanges(TabletServerBatchReader.java:101)
	at org.apache.accumulo.server.manager.state.TabletManagementScanner.<init>(TabletManagementScanner.java:94)
	at org.apache.accumulo.server.manager.state.MetaDataStateStore.iterator(MetaDataStateStore.java:67)
	at org.apache.accumulo.server.manager.state.LoggingTabletStateStore.iterator(LoggingTabletStateStore.java:62)
	at org.apache.accumulo.manager.TabletGroupWatcher.processRanges(TabletGroupWatcher.java:321)
	at org.apache.accumulo.manager.TabletGroupWatcher.hostOndemand(TabletGroupWatcher.java:374)
	at org.apache.accumulo.manager.Manager.hostOndemand(Manager.java:565)
	at org.apache.accumulo.manager.ManagerClientServiceHandler.requestTabletHosting(ManagerClientServiceHandler.java:616)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:569)
	at org.apache.accumulo.server.rpc.HighlyAvailableServiceInvocationHandler.invoke(HighlyAvailableServiceInvocationHandler.java:66)
	at jdk.proxy2/jdk.proxy2.$Proxy33.requestTabletHosting(Unknown Source)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:569)
	at org.apache.accumulo.core.trace.TraceUtil.lambda$wrapService$0(TraceUtil.java:202)
	at jdk.proxy2/jdk.proxy2.$Proxy33.requestTabletHosting(Unknown Source)
	at org.apache.accumulo.core.manager.thrift.ManagerClientService$Processor$requestTabletHosting.getResult(ManagerClientService.java:2399)
	at org.apache.accumulo.core.manager.thrift.ManagerClientService$Processor$requestTabletHosting.getResult(ManagerClientService.java:2375)
	at org.apache.thrift.ProcessFunction.process(ProcessFunction.java:40)
	at org.apache.thrift.TBaseProcessor.process(TBaseProcessor.java:40)
	at org.apache.thrift.TMultiplexedProcessor.process(TMultiplexedProcessor.java:147)
	at org.apache.accumulo.server.rpc.TimedProcessor.process(TimedProcessor.java:50)
	at org.apache.thrift.server.AbstractNonblockingServer$FrameBuffer.invoke(AbstractNonblockingServer.java:492)
	at org.apache.accumulo.server.rpc.CustomNonBlockingServer$CustomFrameBuffer.invoke(CustomNonBlockingServer.java:129)
	at org.apache.thrift.server.Invocation.run(Invocation.java:18)
	at org.apache.accumulo.core.trace.TraceWrappedRunnable.run(TraceWrappedRunnable.java:52)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
	at org.apache.accumulo.core.trace.TraceWrappedRunnable.run(TraceWrappedRunnable.java:52)
	at java.base/java.lang.Thread.run(Thread.java:840)