Prevent run from overriding logging config set in constructor

When creating the `Sanic` instance I provide it with a customized `log_config`.
Calling `run` overrides these settings unless I provide it *again* with the same `log_config`.
This is confusing and error prone. `run` shouldnt override configurations set in the `Sanic` constructor...

sanic/app.py

@@ -545,7 +545,7 @@ class Sanic:
     def run(self, host=None, port=None, debug=False, ssl=None,
             sock=None, workers=1, protocol=None,
             backlog=100, stop_event=None, register_sys_signals=True,
-            log_config=LOGGING):
+            log_config=None):
         """Run the HTTP Server and listen until keyboard interrupt or term
         signal. On termination, drain connections before closing.