Skip param assignment when values is an empty dict

[ytakashina]

With the current implementation of Parameter.setValues(), you will get an exception when you set empty values to a parameter (e.g., when you unit-test your model on some simple data).

        if isinstance(values, dict):
>           indices, values = list(zip(*values.items()))
E           ValueError: not enough values to unpack (expected 2, got 0)

This patch help avoid the exception by skipping assignment when the given values are empty.

[fdabrandao]

Thanks! This corner case had not gone unnoticed.