Allow thread=None in Webhook.send()

Fixes https://github.com/Rapptz/discord.py/issues/10313

discord/webhook/async_.py

@@ -1894,7 +1894,7 @@ class Webhook(BaseWebhook):
         ) as params:
             adapter = async_context.get()
             thread_id: Optional[int] = None
-            if thread is not MISSING:
+            if thread:
                 thread_id = thread.id
 
             data = await adapter.execute_webhook(
@@ -1959,7 +1959,7 @@ class Webhook(BaseWebhook):
             raise ValueError('This webhook does not have a token associated with it')
 
         thread_id: Optional[int] = None
-        if thread is not MISSING:
+        if thread:
             thread_id = thread.id
 
         adapter = async_context.get()
@@ -2102,7 +2102,7 @@ class Webhook(BaseWebhook):
             previous_allowed_mentions=previous_mentions,
         ) as params:
             thread_id: Optional[int] = None
-            if thread is not MISSING:
+            if thread:
                 thread_id = thread.id
 
             adapter = async_context.get()
@@ -2164,7 +2164,7 @@ class Webhook(BaseWebhook):
             raise ValueError('This webhook does not have a token associated with it')
 
         thread_id: Optional[int] = None
-        if thread is not MISSING:
+        if thread:
             thread_id = thread.id
 
         adapter = async_context.get()