Fix an output error for connections costs

[ekatef]

Closes #647

Currently a connection costs file is written by _adjust_capital_costs_using_connection_costs which is called after simplifying links and when removing stubs. If there are no links in the network and remove_stubs: false, a connection costs file is not written which leads to snakemake MissingOutputException error.

Added an output of an empty data frame for connection costs to a short-cut of simplify_links() corresponding to a case without links in the network. In case remove_stubs is called, it happens after simplify_links() and an empty connections costs file will simply be overwriten.

Checklist

• I consent to the release of this PR's code under the GPLv3 license and non-code contributions under CC0-1.0 and CC-BY-4.0.
• I tested my contribution locally and it seems to work fine.
• Code and workflow changes are sufficiently documented.
• Newly introduced dependencies are added to envs/environment.yaml and envs/environment.docs.yaml.
• Changes in configuration options are added in all of config.default.yaml and config.tutorial.yaml.
• Add a test config or line additions to test/ (note tests are changing the config.tutorial.yaml)
• Changes in configuration options are also documented in doc/configtables/*.csv and line references are adjusted in doc/configuration.rst and doc/tutorial.rst.
• A note for the release notes doc/release_notes.rst is amended in the format of previous release notes, including reference to the requested PR.

[ekatef]

Not sure if a release note is needed as that is a simple bug fix

[ekatef]

@davide-f, I think that is ready for review
Actually, it looks like a solution can be much simpler than expected :)