What should I do if I am getting robbed at knifepoint when I don’t have anything on me?

Jake Simmons I hope you don’t mind I added self defense to your question. It will get it more exposure on here.
In self defense world we call this holding at bay. It is when at attacker produces a knife but instead of stabbing you uses it to convince you of another action. In accordance to your question, in this case give over valuables.
You want to convey you don’t have money. If his true motive is robbery he probably will not want to risk a murder or attempted murder charge for no financial gain.
When you do this talk with your hands palms bladed towards you and elbows covering your body. This is designed to make a shield to protect your vitals and neck if he lunges. In addition you can step back ever so slightly at a 30–45 degree angle to give him less access to targets. It has to be subtle and back with just a baby step.
However lets up the ante and pretend he does’t believe your answer and he is still holding you at bay and articulating that if you don’t find valuables on your person right now he is going to stab you. To make matters worse you are cornered back to a wall and you can run away (but maybe past him?)
Normally if you had a shield or barrier that would be a better play. Some barrier to protect you from being stabbed. But due to your question I am going assume you do not. If you cannot run and cannot convince him you don’t have the goods you have two options before he engages.
Push his arm into his body.
Suck his arm into your body.
It can be anywhere from his elbow (best place) to his wrist (worse place but better than his arm being free).
Which one you choose will be contingent upon how much tension is in his arm when you touch it. Not much tension push it into him. Lot of tension suck his arm into you.
The object is to keep his knife arm from moving freely. If you push his arm into him he can’t hit you with the other arm. The shoulders work like a turnstile. One goes forward the other goes back. You may take a punch if you suck the arm into you but it beats being stabbed.
Knee him in the thigh, a couple times and bolt past him. The point of the knees is two fold.
To hurt his leg so he has trouble chasing you and
to give him fear and pain so that he concentrates on that instead of stabbing you.
When you do this (and again this is a before he stabs move) you want to start a sentence and mid-sentence, explode pinning his arm to his body or yours. The idea is that you want him to concentrate on the sentence and not you engaging him.
You are just trying to get away—not win.

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