treat Semaphore._waiters as length zero when it is None (#13638)

We access the `._waiters` private attribute of the Python asyncio
`Semaphore` class.  This was changed in Python 3.10.8 (and other versions)
to be initialized to `None` instead of an empty deque.  Our existing
unconditional length checks failed on the new `None` default.  This seems
to block syncing.

python/cpython#97020

https://github.com/python/cpython/compare/v3.10.7..v3.10.8#diff-0fee1befb15023abc0dad2623effa93a304946796929f6cb445d11a57821e737

Reported traceback:
```python-traceback
2022-10-12T20:03:59.367 full_node full_node_server : INFO Connected with full_node {'host': '65.34.144.6', 'port': 8444}
2022-10-12T20:03:59.370 full_node full_node_server : ERROR Exception: object of type 'NoneType' has no len(), {'host': '65.34.144.6', 'port': 8444}. Traceback (most recent call last):
File "/home/summa/chia-blockchain/chia/server/server.py", line 598, in wrapped_coroutine
result = await coroutine
File "/home/summa/chia-blockchain/chia/full_node/full_node_api.py", line 114, in new_peak
waiter_count = len(self.full_node.new_peak_sem._waiters)
TypeError: object of type 'NoneType' has no len()

2022-10-12T20:03:59.371 full_node full_node_server : ERROR Exception: object of type 'NoneType' has no len() <class 'TypeError'>, closing connection {'host': '65.34.144.6', 'port': 8444}. Traceback (most recent call last):
File "/home/summa/chia-blockchain/chia/server/server.py", line 608, in api_call
response: Optional[Message] = await asyncio.wait_for(wrapped_coroutine(), timeout=timeout)
File "/usr/lib/python3.10/asyncio/tasks.py", line 408, in wait_for
return await fut
File "/home/summa/chia-blockchain/chia/server/server.py", line 605, in wrapped_coroutine
raise e
File "/home/summa/chia-blockchain/chia/server/server.py", line 598, in wrapped_coroutine
result = await coroutine
File "/home/summa/chia-blockchain/chia/full_node/full_node_api.py", line 114, in new_peak
waiter_count = len(self.full_node.new_peak_sem._waiters)
TypeError: object of type 'NoneType' has no len()

2022-10-12T20:03:59.487 full_node full_node_server : INFO Connection closed: 65.34.144.6, node id: 506fe4c05ce6b72bb707471842e552307c7a547aa9ba981175db5c08fa3e47e6
```

Author: altendky

chia/full_node/full_node_api.py

@@ -122,15 +122,16 @@ async def new_peak(self, request: full_node_protocol.NewPeak, peer: ws.WSChiaCon
         """
         # this semaphore limits the number of tasks that can call new_peak() at
         # the same time, since it can be expensive
-        waiter_count = len(self.full_node.new_peak_sem._waiters)
+        new_peak_sem = self.full_node.new_peak_sem
+        waiter_count = 0 if new_peak_sem._waiters is None else len(new_peak_sem._waiters)
 
         if waiter_count > 0:
             self.full_node.log.debug(f"new_peak Waiters: {waiter_count}")
 
         if waiter_count > 20:
             return None
 
-        async with self.full_node.new_peak_sem:
+        async with new_peak_sem:
             await self.full_node.new_peak(request, peer)
         return None
 
@@ -1436,7 +1437,9 @@ async def new_compact_vdf(
         if self.full_node.sync_store.get_sync_mode():
             return None
 
-        if len(self.full_node.compact_vdf_sem._waiters) > 20:
+        compact_vdf_sem = self.full_node.compact_vdf_sem
+        waiter_count = 0 if compact_vdf_sem._waiters is None else len(compact_vdf_sem._waiters)
+        if waiter_count > 20:
             self.log.debug(f"Ignoring NewCompactVDF: {request}, _waiters")
             return None