Treat (0, None) as fixed dim if is the first dimension

merlin/dtypes/shape.py

@@ -153,7 +153,7 @@ def __post_init__(self):
         if self.dims is not None:
             new_dims = []
             for i, dim in enumerate(self.dims):
-                if i == 0 and dim is None:
+                if i == 0 and (dim is None or dim == (0, None)):
                     dim = -1
                 new_dim = Dimension(dim)
                 new_dims.append(new_dim)