leaves: Fix leaf_kwargs None mapping

See also: #4

simple_einet/einet.py

@@ -275,7 +275,7 @@ def _build_input_distribution(self, num_features_out: int):
             num_channels=self.config.num_channels,
             num_leaves=self.config.num_leaves,
             num_repetitions=self.config.num_repetitions,
-            **self.config.leaf_kwargs,
+            **self.config.leaf_kwargs if self.config.leaf_kwargs is not None else {},
         )
 
         return FactorizedLeaf(