Batteries are simply an energy storage device. In order for anything to catch fire it requires heat which is a form if energy. A discharged battery would not contain sufficient energy to create the necessary heat to start a fire. The problem with lithium batteries( the type used in phones and 787s) is that when they are charged they contain a lot of energy relative to their size.

On reflection there is still a potential risk if the device is designed to turn off before the battery is fully discharged, however I suspect the risk may be negligible as most manufacturers will want the maximum run time for their product so are unlikely to shut the device down if there is plenty of charge left.