Wikipedia implies the "original" key is Em, but I have no idea what that's based on. What constitutes "original key", anyway? Again to Wikipedia, so you know it's fully verified and guaranteed accurate, they say it was originally written with Ellafitz in mind but she didn't record it till some years after London did. So is the "original" key the key the songwriter originally set down on staff paper, the key he moved it to for Ella, the key it was moved to for Julie who appears to have made the first recording? Was Ella singing this thing for a year in performances, in a different key from where Julie recorded it? If so, does performance trump recording, or vice versa?
Just play the damn thing in a key where the singer can sing it. If no vocals, play it in any key you want. If most everyone you'd want to play it with knows it in Cm, learn it there. This arguing about what's "original" never gets anyone anywhere. A basic tool of the perfoming musician is the ability to perform songs in different keys to adapt to singers' ranges.