Professor Juniper - Pokémon Rulings Compendium

Can you use Professor Juniper or Professor Sycamore with no cards in your deck, just to discard your hand?: No, you cannot. Discarding your hand is a cost not the effect, and you cannot play Trainer cards for no effect.

Source: TPCi Rules Team (2017-04-27)

Trainers » Professor Juniper | Errata » Professor Juniper | Errata » Professor Sycamore | Trainers » Professor Sycamore

While there are no functional changes to these cards [Professor Juniper and Professor Sycamore], you may not include both of them in your deck. That is, you may have up to 4 copies of Professor Juniper or up to 4 copies of Professor Sycamore - but if you have any Professor Juniper cards in your deck, you may not put any Professor Sycamore cards in your deck, and vice versa.

Source: TPCi Announcement (2013-10-01), TPCi Rules Team (2013-11-21)

Trainers » Professor Juniper

Can I play Professor Juniper if it is the only card in my hand, and then draw 7 cards?: Yes, you basically discard your remaining hand of zero cards, then you draw 7 cards from your deck.

Source: Black & White FAQ (2011-04-28), PUI Rules Team (2011-04-28)