That canister is the carbon canister. I swapped my 77 motor/tranny into my 78 and left that out and have the same tube that isn't connected to anything. That tube is the vapor vent line from the fuel tank. Normally the fumes from the fuel tank are vented into the carbon canister which has activated charcoal to nuturalize the fumes.
I am eventually just going to pull that vent line out of the engine bay and just vent it back by the tank somewhere.
Not having that tube attached to something will not affect the running of the engine, or at least it doesn't on my motor. BUT I would not plug the line up. You could probably vent it into the intake somewhere if you wanted too.
Good luck and let us know what you do with it.
77 280z (wreck 4 parts)
78 280z (Frankenstien created with 95% parts from the 77)