fix flow matching kwargs updating

Author: paul-buerkner

bayesflow/networks/flow_matching/flow_matching.py

@@ -67,11 +67,11 @@ def __init__(
         self.use_optimal_transport = use_optimal_transport
 
         new_integrate_kwargs = FlowMatching.INTEGRATE_DEFAULT_CONFIG.copy()
-        new_integrate_kwargs.update(integrate_kwargs)
+        new_integrate_kwargs.update(integrate_kwargs or {})
         self.integrate_kwargs = new_integrate_kwargs
 
         new_optimal_transport_kwargs = FlowMatching.OPTIMAL_TRANSPORT_DEFAULT_CONFIG.copy()
-        new_optimal_transport_kwargs.update(optimal_transport_kwargs)
+        new_optimal_transport_kwargs.update(optimal_transport_kwargs or {})
         self.optimal_transport_kwargs = new_optimal_transport_kwargs
 
         self.loss_fn = keras.losses.get(loss_fn)