Mounting light on handlebar with bag installed
As Chris H notes, the clerk was wrong. And he suggests a good suggestion to raise the light above the bag. However, you can also mount the light below the bag as well. Operating on the same principle as fog lights on cars, low mounted lights can do a better job of showing some road hazards. They are also much less likely to blind drivers and pedestrians (StVZO-compliant handlebar lights are still sadly rare).

There are lights that will mount on the crown of the front wheel fork (where the bolt for caliper brakes and mudguards attaches), but those might still hit the bottom of the bag -- although the Ortlieb bag shown in Christian Lundvig's photo is quite small and thus compatible with fork crown mount lights. 

Here's a google image search for "bicycle lights fork mount". In addition to the fork crown lights, there are also front fork adapters that let you mount handlebar lights on one of the legs of the front fork. 



Here's a blown up image of the top left two -- a fork-mount adapter and a light mounted on said adapter. Note that this placement is just about where ye' olde friction dynamo lights used to be mounted. 



Another option is of course a helmet light: