`.normalize()` fails for an empty QUBO

[singular-value]

In [38]: qv.QUBO({}).normalize()
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-38-18b132978996> in <module>
----> 1 qv.QUBO({}).normalize()

~.../python3.8/site-packages/qubovert/utils/_dict_arithmetic.py in normalize(self, value)
    781 
    782         """
--> 783         mult = value / max(abs(v) for v in self.values())
    784         for k in self:
    785             self[k] *= mult

ValueError: max() arg is an empty sequence

@jtiosue I'm happy to make a pull request so that this case would simply return the empty QUBO (or PCBO or whatever) back unchanged.

[jtiosue]

Thanks for finding this! It is now fixed on the dev branch. Hopefully soon I will merge dev to master and release a new version.