Select at least two nulls and click Points Follow Nulls. This will generate a new path that will be connected to those nulls using an expression. Tip: by holding shift, you can generate using any selected layer, nulls or not.
Use Static Path from Nulls to generate an expression-free path. This path will initially be shaped according to the selected nulls/layers, but will not follow them as you move them around.
Use additional options like 'Ropes' to make your generated paths bend when your nulls get closer to each other, or 'Path Closed' to quickly toggle the path closed/open feature on and off.