Captain reported an APU auto shut down in flight with one IDG already on MEL. The flight crew performed a diversion and made a precautionary landing.

Narrative

Approximately 15 minutes after top of climb with MEL 24-XX-XX GEN already deferred; the APU auto shutdown. We received L and R UTIL BUS OFF; C HYD ELEC 2 EICAS messages. We attempted to restart the APU and then got an APU OIL QTY status message and the oil quantity indicated zero on the status page. We ran the QRH for the utility bus off messages and discussed our single generator status. I sent a message to Dispatch informing them of the situation and that ZZZZ looked like the best option to us. Dispatch concurred and we coordinated with ATC to return to ZZZZ and [requested priority handling]. We declined the need for assistance on the ground and taxied to our Gate under our own power. APU oil quantity verified to be zero; found by mechanic post flight when he physically verified it in the APU compartment. The mechanic said he did not see any oil leaking in or around the compartment. The APU didn't register the status message or show low oil quantity before we departed ZZZZ1. It seems to be a failure of the APU and no human interaction could have prevented it.