[Bug] Required Images for ImageSlider

Since the latest Change ImageSlider.vue takes a required images-Prop. Now the compiler throws up some Errors, because of potentially undefined images.

AK:

• The ImageSlider always gets at least an empty images-array as prop
• Revert deletePhoto()-Prop to optional (it is not always intended that the user has the option to delete photos manually) and adapt imageSlider component accordingly