LArRecUtils: Don't complain in LArADC2MeVCondAlg if low gain data are missing.

In one configuration, observed errors from LArADC2MeVCondAlg because
low gain data were missing.  With LArADC2MeVTool, this would not be
an error.  Update LArADC2MeVCondAlg so that this is not an error.