fix(core.py): handle None return value from run() in start() function

Ensure that the start() function returns 0 when run() returns None.
This change prevents potential issues where a None return value
could cause unexpected behavior or errors in parts of the system
that expect an integer return code.

Author: kzndotsh

tux/cli/core.py

@@ -235,7 +235,8 @@ def start() -> int:
 
     from tux.main import run
 
-    return run()
+    result = run()
+    return 0 if result is None else result
 
 
 # Ensure commands are registered when this module is imported