Fix for DFT+U commutator with single mesh function
Description
Fixes to the DFT+U commutator for single mesh function, in non-default codepath.
News snippet
Minor bugfix for the electronic current from DFT+U, in non-default codepath.
Checklist
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I have checked that my code follows the Octopus coding standards -
I have added tests for all the new features added in this request.