Weedkillers for soft green growth are divided between those that will kill through the leaves, any type of leaf, and those that kill in the same way but leave lawn grass largely unharmed. There is no formulation that will leave your catnip untouched - its just another broader leaved plant to a weedkiller.
The area looks to be quite small and there isn't a huge amount of exposed soil -weeding by hand using a hand fork or border fork wouldn't take very long, if you are able to do it. Alternatively, you can just apply weedkiller such as glyphosate to the green growth of plants/weeds you don't want, taking care not to get any on those you do.