Prevent segfault for bad group name

Users who had a mismatch between the group name when the group is configured in the python and the group name when the fill() function is called. The existing check for an empty was not being used, since the [] operator on the GaudiHandleArray was returning a nullptr for non-existant elements of the array. I now check for a nullptr and offer a useful warning.

Users are now presented with the following.

FATAL The tool ExampleMonitor1 could not be found in the monitoring algorithm's tool array. This probably reflects a difference between your python configuration and c++ filling code.

Resolution for https://its.cern.ch/jira/browse/ATLASDQ-713.