As John said, there is nothing to rasterize. Pixels are pixels. At most you could convert the layer styles to individual layers, but that's it. There is no specific option to flatten them into the currently active layer, so merging with a blank layer is the only way to do that.
I am guessing that when you made your rectangle you used the Fill Pixels (right) option. If you had used the Shape Layer (left ) option to create the rectangle you would have had something to raterise.