i have a table of contents like this:
I wanna have an empty line between the topics.
Can i teach InDesign to insert an automatic line break?
Maybe by paragraph styles? Would be easy here, because the style could insert a line break
before the text !everytime the style will be used.
As part of the paragraph style you define for the topic heading (which will include all the character formatting information for the heading) you can add a Space Before value in the Indents and Spacing section. This will add the space of your choice without need for any empty paragraph that you would get by adding a line break (which you cannot do with a style).
Imho, better to insert a space before in the para style!