The contrast between occupied and empty houses is evident on one block, where high grass in weedy clumps gives way to a neatly mowed lawn with handwritten signs pleading Please do not let your dog poop on our yard.Homeowner Norma Hernandez, one of the few people outside on a recent sunny afternoon, can point out which families are permanent on her block.Rented, owned, rented, rented, rented, s