config: handle empty or missing MicroflowConstants

If the mxruntime/MicroflowConstants option is completely missing in the
config, this code would explode on the dictionary index.

If the option is present, but has no value specified as all, it ends up
as None, and causes an "AttributeError: 'NoneType' object has no
attribute 'items'" a few lines later, when asking for .items()

So, handle both scenarios in a better way.

Github: Fixes #39

src/m2ee/config.py

@@ -607,7 +607,9 @@ def get_constants(self):
         model_constants = {}
         for metadata_constant in self._model_metadata['Constants']:
             model_constants[metadata_constant['Name']] = metadata_constant['DefaultValue']
-        yaml_constants = self._conf['mxruntime']['MicroflowConstants']
+        yaml_constants = self._conf['mxruntime'].get('MicroflowConstants', None)
+        if yaml_constants is None:
+            yaml_constants = {}
         constants_to_use = {}
         default_constants = {}
         for name, value in model_constants.items():