[networking] Fix POST requests with zero-length payloads (#7648)

Bugfix for 227bf1a33b

Authored by: bashonly

yt_dlp/networking/_urllib.py

@@ -315,7 +315,7 @@ def get_method(self):
 def update_Request(req, url=None, data=None, headers=None, query=None):
     req_headers = req.headers.copy()
     req_headers.update(headers or {})
-    req_data = data or req.data
+    req_data = data if data is not None else req.data
     req_url = update_url_query(url or req.get_full_url(), query)
     req_get_method = req.get_method()
     if req_get_method == 'HEAD':

yt_dlp/networking/common.py

@@ -425,7 +425,7 @@ def headers(self, new_headers: Mapping):
             raise TypeError('headers must be a mapping')
 
     def update(self, url=None, data=None, headers=None, query=None):
-        self.data = data or self.data
+        self.data = data if data is not None else self.data
         self.headers.update(headers or {})
         self.url = update_url_query(url or self.url, query or {})