OR 324 is quite the right article for this. If the employer doesn't want that you work (because he has nothing to do for you) it's his problem, not yours. He still has to pay if you are there and ready to do work. This is for instance also mentioned in this article.
The meaning of this is obvious if having a contract with a fixed number of work hours per day/month/year.
Prove that you have a fixed work contract.
If the shift plans are made in advance you have a proof that you have a certain number of hours to work (and thus an expected income). If I interpret this here correctly, this is "echte Arbeit auf Abruf" (true work on request), because if your employer wants your work, you have to be there according to the shift plan, as opposed to your employer asking "who is ready to work tonight?".
So your employer must pay you the hours agreed on in the shift plan, regardless of whether he has work for you or not (but you must explicitly tell him that you are willing to take work).
Can the employer change the shift plan, and to what extent?
The employer must announce changes to work hours as soon as possible, and changes on short notice are only acceptable in emergency cases. A reduction in work hours due to not enough work shall not reduce the employee's salary. The business risk is entirely with the employer and he must not shift that responsibility to his employees. (That was common in the late 19th and early 20th century, with all the officially self-employed home workers in the textile industry). Here is a federal court decision that affirms this (BGE 125 III 65 S. 66).