[lldb] [llgs] Fix assertion in Handle_qfThreadInfo (#88301)

Currently, GDBRemoteCommunicationServerLLGS::Handle_qfThreadInfo asserts
if the number of processes under debug isn’t 1 and the multiprocess
feature isn’t supported. This is so that we don't string IDs of threads
belonging to different processes together without including the IDs of
the processes themselves in the response when there are multiple
processes under debug. However, it’s conceivable that we have no process
under debug and the multiprocess feature isn’t supported. So, have
GDBRemoteCommunicationServerLLGS::Handle_qfThreadInfo assert if the
number of processes under debug is greater than 1 and the multiprocess
feature isn’t supported.