Funny this thread should come up - I was just wondering this the other day. I’ve mostly called it ESB, probably because that seems to be the more common abbreviation, but realise this is inconsistent with the other films. For example, people tend to include the “T” in “TPM”, “TFA”, “TLJ” and “TROS”.
If searching on Google for things related to Empire, “ESB” is far more lucrative.