Initialise the read handle in GenBase (ATEAM-813)

The GenBase class has started throwing errors due to !48861 (merged) because it has a single ReadHandle, uninitialised.

This MR fixes the initialisation, but also tidies up some other legacy behaviour around the handle, including converting it to a handlekey. The public accessor method for the handle was instantiating a new one each time, whereas now does the SG::makeHandle step, but given the way it is used by clients there might be a need for a wider tidy. Example: https://acode-browser1.usatlas.bnl.gov/lxr/source/athena/Generators/GeneratorFilters/src/MultiBjetFilter.cxx#0107