Fix condition oct diel func

Description

Fix condition for the utility oct-dielectric-function for transient absorption, if the variable GaugeFieldDelay was not present, but TransientAbsorptionReference was there.

News snippet

Fix condition for the utility oct-dielectric-function for transient absorption, if the variable GaugeFieldDelay was not present, but TransientAbsorptionReference was there.

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.