Fix MACE model (#99)

* Fix default model so more useful error is displayed

* Fix comment

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Co-authored-by: ElliottKasoar <ElliottKasoar@users.noreply.github.com>

janus_core/mlip_calculators.py

@@ -51,9 +51,9 @@ def choose_calculator(
         from mace.calculators import MACECalculator
 
         # `model_paths` is keyword for path to model, so take from kwargs if specified
-        # Otherwise, take `model` if specified, then default to `None`, which will
+        # Otherwise, take `model` if specified, then default to `""`, which will
         # raise a ValueError
-        kwargs.setdefault("model_paths", kwargs.pop("model", None))
+        kwargs.setdefault("model_paths", kwargs.pop("model", ""))
         kwargs.setdefault("default_dtype", "float64")
         calculator = MACECalculator(device=device, **kwargs)