Investment setting of pipes with `nonconvex=0` is not working

**Describe the bug**
If the investment tables contains an row with a heatpipeline type with `nonconvex=0`, this does not work with the current oemof-solph version, as in the meanwhile, an error is raised in oemof-solph, if there are fixed_investment costs and nonconvex is set to false. (Before the fixed cost were ignored.)