My 790's handles are solid. You probably don't need to remove them to determine that. Look for a parting line seam which should run full length of the grab handle.
To make them hollow when molding, there would have to be some kind of core in the mold (to keep the plastic out). Then, that core would have to be removed. The only way I can see that being done is by molding two halves and gluing those halves together. That would add cost. Deere doesn't like adding cost.
The attaching studs are probably molded right into the handle, btw.
Now, the grab handles on the 4200-4400 tractors are quite a bit longer then the handles on the 790, but you should have a good idea of what to look for.