Here is Serebii's chart:
It appears that the neutral natures — Hardy, Quirky, Serious, Bashful, and Docile — do not have an Malasada that affects them, unfortunately. If you want Type: Null's friendliness to go up quickly, give it massages. You could also give it the Soothe Bell and run around on Tauros for a long time. You can also feed it friendship-increasing Berries if you don't mind the stat drop.