TOSQUASH: debug mode loglevel

tractor/_root.py

@@ -127,8 +127,7 @@ async def open_root_actor(
             "Debug mode is only supported for the `trio` backend!"
         )
 
-    loglevel = log._default_loglevel = loglevel.upper()
-    assert loglevel
+    loglevel = (loglevel or log._default_loglevel).upper()
     log.get_console_log(loglevel)
 
     # make a temporary connection to see if an arbiter exists