[common] prevent exception when using empty 'user-agent' (#8116)

use '"headers": {"User-Agent": ""}' to send an empty User-Agent header

gallery_dl/extractor/common.py

@@ -462,7 +462,7 @@ class Extractor():
                 headers["Referer"] = self.root + "/"
 
         custom_ua = self.config("user-agent")
-        if custom_ua is None or custom_ua == "auto":
+        if not custom_ua or custom_ua == "auto":
             pass
         elif custom_ua == "browser":
             headers["User-Agent"] = _browser_useragent(None)