[DTWIN-3749] Fix unbound local error when multiplying empty array

[lincrosenbach]

>>> from barril.units import Array
>>> import numpy as np
>>> Array(np.array([]), 'm') * 2
Array(length, [], m)
>>> Array([], 'm') * 2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "~/Work/miniconda/envs.25/alfatwin_stack-py310/lib/python3.10/site-packages/barril/units/_array.py", line 360, in __mul__
    return self._DoOperation(self, other, "Multiply")
  File "~/Work/miniconda/envs.25/alfatwin_stack-py310/lib/python3.10/site-packages/barril/units/_array.py", line 429, in _DoOperation
    return self.__class__.CreateWithQuantity(q, result)  # type:ignore[return-value]
UnboundLocalError: local variable 'q' referenced before assignment

[nicoddemus]

This change also warrants a changelog entry, I believe.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[nicoddemus]

Thanks, and sorry for all the nitpicking!

[nicoddemus]

Sorry, missed this during review, but FYI:

When creating new changelog entries, use UNRELEASED for the date like this:

Suggested change

	*2026-01-14*
	*UNRELEASED*

This way it is clear this release was not done yet when looking at master, and this is naturally updated to the actual release date as part of the release process.