update(MFSimulationBase): allow simulations to have no attached models

Author: jlarsen-usgs

flopy/mf6/mfsimbase.py

@@ -805,6 +805,9 @@ def load(
                 package="nam",
                 message=message,
             )
+        if models is None:
+            return instance
+        
         for item in models:
             # resolve model working folder and name file
             path, name_file = os.path.split(item[1])