Fix (dask) meta handling in spell functions (closes #242)

climix/index_functions/spell_functions.py

@@ -36,6 +36,7 @@ class SpellLength(ThresholdMixin, ReducerMixin, IndexFunction):
             axis=-1,
             dtype=int,
             concatenate=False,
+            meta=np.array((), dtype=int),
         )
         res = da.ma.masked_array(
             da.ma.getdata(res), np.broadcast_to(mask[..., np.newaxis], res.shape)
@@ -44,7 +45,7 @@ class SpellLength(ThresholdMixin, ReducerMixin, IndexFunction):
 
     def chunk(self, raw_data, axis, keepdims, computing_meta=False):
         if computing_meta:
-            return np.array((0,), ndim=1, dtype=int)
+            return np.array((), dtype=int)
 
         data = self.condition(raw_data, self.threshold.points)
         chunk_res = self.kernels.chunk(data)