BUG: pysat.utils.listify breaks for empty list input []

[rstoneback]

Describe the bug
A clear and concise description of what the bug is.

To Reproduce
Steps to reproduce the behavior:

1. pysat.utils.listify([])
2. See error

From unit testing

iterable = []

    def listify(iterable):
        """Returns a flattened list of iterable if not already a list
    
        Parameters
        ----------
        iterable : iter-like
            An iterable object that will be wrapped within a list
    
        Returns
        -------
        list
            An enclosing 1-D list of iterable if not already a list
    
        """
    
        arr_iter = np.asarray(iterable)
        if arr_iter.shape == ():
            list_iter = [arr_iter.tolist()]
        elif arr_iter.shape[0] >= 1:
            list_iter = arr_iter.flatten().tolist()
    
>       return list_iter
E       UnboundLocalError: local variable 'list_iter' referenced before assignment

Expected behavior
Should work fine.

Screenshots
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Desktop (please complete the following information):

• OS: macOS
• Version python 3.7
• Other details about your setup that could be relevant

Additional context
As is, code will break on any input that doesn't match the two checks.