It seems to me that there is no such thing as absolutely zero resistance, because there is a level of antibiotic concentration so low that any organism can strictly survive and reproduce in it’s presence. It comes in degrees.
To my knowledge, all organisms have some very low level of intrinsic resistance that makes them able to survive in the presence of some antibiotic, provided the concentration is low enough. That concentration might be as low as one molecule pr. cell, or perhaps even lower.
But that implies that all mutations that affect antibiotic resistance merely tune it up or down.
The same would be true for basically any poison, toxin, pollutant, or what have you. There is some concentration of nervegas so low you can survive it, some level of radioactivity so low it doesn’t measurably affect you, some concentration of snake-venom so low it won’t affect you, etc. etc.
Now of course, that just means if you want to test for whether spontaneous mutants yield increased resistance, you use a concentration of antibiotic large enough that it normally kills all cells without those mutations. That way you know, since the only difference between the cells that die and the cells that survive, is those mutations, then that is the only (sensible) explanation for the increased resistance.