Can 0 or negative power creatures deal direct damage?
Magic: the Gathering cards usually mean exactly what they say, nothing more and nothing less.

The power of a creature is only relevant when


Dealing combat damage
An effect explicitly mentions the power of that creature (albeit maybe hidden behind keywords, like "fight")


The ability granted by Viridian Longbow doesn't mention the power of the equipped creature, so therefore it doesn't care. One damage is dealt regardless.