Fix: skip loading unitialized model from state

This is due to the fact that `TransformedTargetRegressor` receives a `regressor` as input, then clones it for every `fit` and sets it to `regressor_`, leaving the original `regressor` uninitialized forever.

gordo/machine/model/models.py

@@ -193,7 +193,7 @@ def __getstate__(self):
         return state
 
     def __setstate__(self, state):
-        if "model" in state:
+        if "model" in state and state["model"] is not None:
             with tempfile.NamedTemporaryFile("wb", suffix=".keras") as tf:
                 tf.write(state["model"])
                 state["model"] = load_model(tf.name, compile=False)