fix in add_heating_capacities_installed_before_baseyear to account for case when there is no rural heating demand for some nodes in network.

Closes # (if applicable).

Changes proposed in this Pull Request

if-else check in list-comprehension on l.444 to avoid error because rural heating demand does not exist for that region, i.e. is zero.

Checklist

• I tested my contribution locally and it seems to work fine.
• Code and workflow changes are sufficiently documented.
• Changed dependencies are added to envs/environment.yaml.
• Changes in configuration options are added in all of config.default.yaml.
• Changes in configuration options are also documented in doc/configtables/*.csv.
• A release note doc/release_notes.rst is added.