How to create a symbol with no fill color in qgis?

Set the alpha channel of the fill color to 0.

in the symbol properties, you can change the symbol type from "Fill" to "Border: simple line". This way you do not have a fill. (I am sorry if these are not the exact name, but I am translating from a non English version. However I'm sure that you can find it)

you can just make the opacity of the stroke color zero