1) You are putting it on me to tell you that I need something, right in the middle of my need. 2) If you're my real friend you don't need to say that, I already know I can call you 24/7/365 3) Your willingness to help is nulled by your lack of initiative.
The best thing anyone ever did was call me and say "I'm at Costco. What do you need? And don't say 'nothing'. I'm going to bring you something, and that's that." That whole pizza and frozen burritos were bad for me, but the last thing I needed on my mind at that time was cooking. And that's what true friends are there for.
So don't say "If you need anything let me know!" It's borderline passive aggressive.