prevent crash when client sends empty event

src/socketio/async_namespace.py

@@ -29,7 +29,7 @@ class AsyncNamespace(base_namespace.BaseServerNamespace):
 
         Note: this method is a coroutine.
         """
-        handler_name = 'on_' + event
+        handler_name = 'on_' + (event or '')
         if hasattr(self, handler_name):
             handler = getattr(self, handler_name)
             if asyncio.iscoroutinefunction(handler) is True:
@@ -194,7 +194,7 @@ class AsyncClientNamespace(base_namespace.BaseClientNamespace):
 
         Note: this method is a coroutine.
         """
-        handler_name = 'on_' + event
+        handler_name = 'on_' + (event or '')
         if hasattr(self, handler_name):
             handler = getattr(self, handler_name)
             if asyncio.iscoroutinefunction(handler) is True:

src/socketio/namespace.py

@@ -21,7 +21,7 @@ class Namespace(base_namespace.BaseServerNamespace):
         method can be overridden if special dispatching rules are needed, or if
         having a single method that catches all events is desired.
         """
-        handler_name = 'on_' + event
+        handler_name = 'on_' + (event or '')
         if hasattr(self, handler_name):
             return getattr(self, handler_name)(*args)
 
@@ -152,7 +152,7 @@ class ClientNamespace(base_namespace.BaseClientNamespace):
         method can be overridden if special dispatching rules are needed, or if
         having a single method that catches all events is desired.
         """
-        handler_name = 'on_' + event
+        handler_name = 'on_' + (event or '')
         if hasattr(self, handler_name):
             return getattr(self, handler_name)(*args)