[LLDB] Minidumps without Threads never report as Stopped

This was accidentally discovered on my recent patch and is a corner case. Minidumps without threads are never set to stopped and so many commands don't work. This might seem like an illogical workflow, but it would be acceptable to have a Minidump with no threads and just be a memory dump.

Additionally, I didn't verify this but ProcessElfCore also walks it's threads to get stop reasons, so this likely also affects ProcessElfCore.