Regression in #519a4f2: Adding `p_max_pu` for `conventional_carriers` through `.csv` file

Checklist

• I am using the current master branch or the latest release. Please indicate.
• I am running on an up-to-date pypsa-eur environment. Update via conda env update -f envs/environment.yaml.

Describe the Bug

(currently migrating from version 0.8.0 to current master)

With commit #591a4f2 and specifically here

https://github.com/PyPSA/pypsa-eur/blob/d7554bccffd02933cbe2c9d129b7ae006645add6/rules/build_electricity.smk#L331-L342

providing a filename to a data/*.csv file in the config like

https://github.com/PyPSA/pypsa-eur/blob/d7554bccffd02933cbe2c9d129b7ae006645add6/config/config.default.yaml#L228-L229

only works if the conventional_carrier is also specified here:

https://github.com/PyPSA/pypsa-eur/blob/d7554bccffd02933cbe2c9d129b7ae006645add6/config/config.default.yaml#L113

but that also causes existing powerplants to be included.

Before it was possible to not have existing powerplants included but allow for e.g. conventional power plants via

https://github.com/PyPSA/pypsa-eur/blob/d7554bccffd02933cbe2c9d129b7ae006645add6/config/config.default.yaml#L105

to have those types extendable and put in p_max_pu via the config line above linking to data/*.csv files.

Not sure if that was intentionally changed, but it seems like it is now no longer possible to have p_max_pu through .csv file + no existing powerplant capacities + extendable powerplant capacities via config.

Error Message

(weird failures, because the filename is added as p_max_pu instead of the values.

Start with rules/build_electricity.smk at the linked lines, then compare the related conventional_carriers and conventional_carrier settings in config/config.default.yaml. Reproduce the configuration using a data/*.csv p_max_pu file, without existing powerplant capacities, and verify that the CSV values are used while the intended powerplant types remain extendable.