Closes # (if applicable).
This PR fixes a bug for cases with multi-decade optimisation, activated transmission limit and empty list of lines or DC links.

Changes proposed in this Pull Request

if the set of extendable lines or DC links is empty the loop is continued

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• Newly introduced dependencies are added to environment.yaml, environment_docs.yaml and setup.py (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.