Fix small issues in add_land_use_constraint_m

[tgi-climact]

Changes proposed in this Pull Request

Fix p_nom_min of renewables generators for myopic approach and add check of existing capacities in add_land_use_constraint_m. This reproduce the behaviour in add_land_use_constraint.

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.

[lisazeyen]

@tgi-climact thank you!