Fix provenance issue in pv_capacity_factor.R

[katjaweigel]

Remove empty field from provenance.

Description

There is an empty field in the provenance in pv_capacity_factor.R which needs to be removed because it creates an error with the new core.

• Closes one small point in Prepare release 2.4.0 #2354
• Link to documentation:

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[zklaus]

Straight-forward syntax fix. Thanks, @katjaweigel.