In real life any street cop in .de is happy with you having any light at all, when riding a bicycle in the dark. At least that was my experience there. The exception may be when they are doing a special action check for a day, or so, across the whole city. Then you may be fined, cited, or even get some negative points in 'Flensburg'/your drivers license.

But that applies more to running red lights, or using bicycle lanes in the wrong direction.