Operations with "empty" Arrays can cause UnboundLocalError

[prusse-martin]

• Barril version: 1.10.0 (build py_0 from conda-forge)
• Python version: Python 3.6.7 (build he025d50_1008_cpython from conda-forge)
• Operating System: Microsoft Windows 10 Pro (10.0.19041 N/A Build 19041)

Description

Operating with empty Arrays (values are an empty list or tuple) barril will raise an UnboundLocalError.
When using an empty numpy.ndarray as values there is no error and an empty Array is returned (this is the workaround we are using, force the relevant Arrays to hold numpy.ndarrays).

What I Did

>>> from barril.units import Array
>>> Array([1, 2], 's') / 2
Array(time, [0.5, 1.0], s)
>>> 2 / Array([1, 2], 's')
Array(1 / time, [2.0, 1.0], 1/s)
>>> Array([1], 's') / 2
Array(time, [0.5], s)
>>> Array([], 's') / 2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "W:\alfasim\envs\_alfasim_gui-win64-py36\lib\site-packages\barril\units\_array.py", line 286, in __truediv__
    return self._DoOperation(self, other, "Divide")
  File "W:\alfasim\envs\_alfasim_gui-win64-py36\lib\site-packages\barril\units\_array.py", line 360, in _DoOperation
    return self.__class__.CreateWithQuantity(q, result)
UnboundLocalError: local variable 'q' referenced before assignment
>>> 2 / Array([], 's')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "W:\alfasim\envs\_alfasim_gui-win64-py36\lib\site-packages\barril\units\_array.py", line 302, in __rtruediv__
    return self._DoOperation(other, self, "Divide")
  File "W:\alfasim\envs\_alfasim_gui-win64-py36\lib\site-packages\barril\units\_array.py", line 360, in _DoOperation
    return self.__class__.CreateWithQuantity(q, result)
UnboundLocalError: local variable 'q' referenced before assignment
>>>