profiling(Gecko): check if thread list is not empty before accessing it

It seems to sometimes happen that some samples (possibly at the very start / end)
do not have any active threads in the interpreter state.

It is OK to make main_tid maybe None here, since if it is, the threads list is
empty, and so the for loop will not execute anyways.

Signed-off-by: Sofia Donato Ferreira <flowlnlnln@gmail.com>

Author: flowln

Lib/profiling/sampling/gecko_collector.py

@@ -173,7 +173,9 @@ def collect(self, stack_frames, timestamps_us=None):
         for interpreter_info in stack_frames:
             # Since 'threads' is in order from newest to oldest,
             # we know the first thread must be the main thread.
-            main_tid = interpreter_info.threads[-1].thread_id
+            main_tid = None
+            if len(interpreter_info.threads) != 0:
+                main_tid = interpreter_info.threads[-1].thread_id
             for thread_info in interpreter_info.threads:
                 frames = filter_internal_frames(thread_info.frame_info)
                 tid = thread_info.thread_id