In her book Creating Mandalas with Sacred Geometry Susanne F. Fincher describes the use of empty space in Islamic art:
Emptiness, the void, is a metaphor for the all-encompassing unity of God. This belief is expressed by designs where careful attention is given to the negative spaces around lines and forms
I have used empty spaces in my mandalas in various ways. In general I like to use color to enhance the geometric design and any empty space is incorporated into that design. In other words the empty spaces are an intentional part of the mandala as opposed to something that has been left unfinished.
A fancy doodle, much like embroidered scarves I worked on with needle and thread when I was growing up. But, this was done with Tombow water color markers.
Sometimes an outlined area suggests scaffolding or possibilities, what is not yet manifest.
One of the benefits of coloring mandalas is that it is an easy activity that can absorb our attention. I find this is especially true when I am working with a design that has intricacy and simplicity at the same time.