In continental EU legislation (AFAIK unlike USA), no contract can waive rights provided by legislation.
If a job contract includes provisions that explicitly give up some of basic job limits (say, no vacation, unpaid overtime, limits of shift/week length, etc) - then, barring very few exceptions (listed in the same legislation), those parts of agreement are null and void. It's the same as it's everywhere for minimal wage - even if you sign a contract that you'll work for $0.05/hour, then that provision anyway means $[minwage]/hour.
If a job contract includes provisions that explicitly give up some of basic job limits (say, no vacation, unpaid overtime, limits of shift/week length, etc) - then, barring very few exceptions (listed in the same legislation), those parts of agreement are null and void. It's the same as it's everywhere for minimal wage - even if you sign a contract that you'll work for $0.05/hour, then that provision anyway means $[minwage]/hour.