Whenever you finish a long rest, you can touch one weapon that you are proficient with and that lacks the two-handed property.
So let's say I wake up and don't choose a weapon. Then, later on, I get into a fight and summon a Shadow Blade. Then I go to myself, "Yah know what, I'm touching this weapon!"
Would you then get to add the benefits of the Hex Warrior ability?
Given that it automatically applies to any Pact Weapon you summon, it's not like you'd be without a weapon during the rest of the day either, even after the Shadow Blade has disappeared.