Johnny Hardwick's body was in bad shape when he died -- so much so that officials haven't been able to figure out how exactly he died ... this according to the medical examiner. According to the 'King of the Hill' star's autopsy report, obtained by TMZ, Johnny was found dead in his bathtub back in August -- and the water was running when he was found ... but the drain was open and he was not actually submerged. The M.E. says there were no obvious signs of trauma ... adding that a toxicology test showed a "presumptive positive test for...