code: protect access to optional attribute

Signed-off-by: Christian Hopps <chopps@labn.net>

munet/base.py

@@ -1237,7 +1237,7 @@ def run_in_window(
             envvars = f"MUNET_NODENAME={self.name} NODENAME={self.name}"
             if hasattr(self, "rundir"):
                 envvars += f" RUNDIR={self.rundir}"
-            if self.unet.config_dirname:
+            if hasattr(self.unet, "config_dirname") and self.unet.config_dirname:
                 envvars += f" CONFIGDIR={self.unet.config_dirname}"
             elif "CONFIGDIR" in os.environ:
                 envvars += f" CONFIGDIR={os.environ['CONFIGDIR']}"