If you're referring to the "Memory Usage (Dedicated)" in the "Sensors" tab of GPU-Z, I think you are thinking of something different. Mine shows a null value as well, but this is normal. The memory size you are thinking of is in the "Graphics card" tab and is the "Memory size".
The "dedicated" memory in GPU-Z is the amount of system (non-GPU) memory that is reserved for use by the graphics card (whether or not it is needed), whereas the "Dynamic" memory is the amount of memory that is actually being used by the graphics card at any given time for textures etc.. With DirectX 9 and before, all data in graphics memory was required to be duplicated in system memory and thus there was "Dedicated" system memory for graphics, usually equivalent to the memory capacity of the graphics card but sometimes bigger. With DirectX 11 and above, system memory is allocated for graphics dynamically and there is no "Dedicated" memory. Furthermore in DX11 and above there is no requirement that the the graphics memory be replicated in system memory, so the only time system memory is used is when the GPU runs out of graphics memory. This frees up a lot of system memory and is the reason why there is no "Dedicated" memory for you or any system running DX11.
This link also gives a good explanation of this.
I hope this clears things up.
