add EU oil bus already when adding land transport to be able to run t…

[lisazeyen]

• add EU oil bus already in function add_land_transport(n) to avoid warning of missing bus for loads and being able to run the model without the industry sector
• when removing todays industrial electricity demand by scaling down total eletricity demand, remove "RuntimeWarning: invalid value encountered in double_scalars" by checking if the load is empty (for not dividing by zero)

[fneum]

Accept as a hotfix. For the long-term, we could think of a function that initially adds all required buses depending on sectoral coverage.