lib.log: Fix mypy.possibly-undefined fallout

The log_m() function has margs conditionally assigned inside an if
block but used unconditionally afterwards. Initialize margs outside
the if to guarantee it is always defined.

Assisted-by: unsloth/Qwen3.6-35B-A3B-GGUF:IQ4_NL and pi.dev 0.81.1
Signed-off-by: Jan Lindemann <jan@janware.com>

src/python/jw/pkg/lib/log.py

@@ -175,8 +175,8 @@ def get_caller_pos(up: int = 1,
 def log_m(prio: int, *args, **kwargs) -> None:  # export
     if prio > _level:
         return
+    margs = ''
     if len(args):
-        margs = ''
         for a in args:
             if isinstance(a, list):
                 margs += '\n'.join([str(elem) for elem in a])