D3D11 debug layer warning is printed when having a vertex shader with no inputs

Not having any inputs in a vertex shader is perfectly valid, as is exercised by Quick 3D particles now. With D3D11 creating an input layout object with 0 elements leads to a STATE_CREATION WARNING #420 debug layer warning, when enabled (i.e. when running with QSG_RHI_DEBUG_LAYER=1). This should be handled in QRhi's D3D11 backend by skipping the input layout object creation as appropriate, and rather calling IASetInputLayout with nullptr if there are no inputs.