Relevance of equilibrium constant Kp for reaction when reactant is solely a solid and is absent

Consider a reaction where a solid dissociates to give gases. There must a equilibrium between them, define the equilibrium constant in terms of pressure as $K\_p$. Suppose I take the gases alone in a container without the solid. The amount is such that the $Q\_p$ comes out to be less than $K\_p$. Now would they show back reaction?


While being introduced to concept of $K\_c$ we are told concentration of solids is constant so it isn't considered in equilibrium constants. But here the concentration is altogether 0, so why won't/how will we take that into consideration.


