Actually it isn't! An empty stomach has the highest acid levels whilst a full stomach has the lowest and gives bacteria the best chance. A full stomach will have a pH of 4 to 7 depending upon the food, whilst am empty stomach can be 0.8 to 2 which will kill most probiotics.
Consuming probiotics with fruits or better still vegetables is the best idea because acid levels will be lowest and the soluble fibre will provide a food source for the bacteria.