Fixed bug where the default unit was "m" instead of sending None

When the log_activity function receives a value of None for the unit parameter, it logs value without applying a conversion multiplier.

fitnessCog/fitness.py

@@ -117,7 +117,7 @@ class FitnessCog(commands.Cog):
                 )
                 return
 
-            unit = parts[2] if len(parts) > 2 else "m"
+            unit = parts[2] if len(parts) > 2 else None
 
             timestamp = int(message.created_at.timestamp())
             username = message.author.name