fastboot: Check if super_empty.img exists before reading it.

On non-DAP devices, fastboot flash <partition> will spam error messages
about not being able to read super_empty.img. We should check that
super_empty.img exists before trying to read it.

Bug: 120429424
Test: fastboot flash system on non-DAP device
Change-Id: I6c4eec19cb3ef8d24595a75e072e1d75baaa8cdd