Running VM in suspend=n mode might return VM_START including a NULL thread.

[lateralusX]

This is mainly in embedding scenarios not running regular Main method and setting up debugger agent in suspend=n mode. If the embedder hasn't explicitly setup a main thread using mono_thread_set_main API, the VM_START returned to debugger client on attach will include a NULL thread object and when debugger client asks for more information around this object, runtime will shutdown with an error, since it consider the object to be garbage (not existing in this case).

This problem in debugger client is hard to track for embedders so we should at least handle the scenario gracefully with either an error message or handle/recover from the case where VM_START returns a NULL thread object.