Select your shape layer in the Timeline and then press the U key twice to reveal the Shape > Path and Stroke > Color and Width property.
Enable keyframing by clicking the stopwatch next to the path (probably Path 1).
Press the G key to select the pen tool
Press the Cmnd / Ctrl key to select the vertex you want to move and animate. You can't select vertex points of shape layers with the direct selection tool (v). You can only manipulate them when you have the Pen tool (g) selected. The modifier key (cmnd or apple key on a mac, or ctrl key on windows) allows selection of a vertex, alt/option for convert vertex to change the bezier handles.
There you go.
I'm sorry. i should have explained it a little bit better.
i wanted to keyframe it but at the same time i want one end point to be parented to one null while the other endpoint to the other null
You can't access individual vertex points with an expression. There are some 3rd party plug-ins that will help and there are some scripts that can attach tracker info to individual vertex points so they could be used to link your null positions to the start and end vertex. These scripts create keyframes so you would have to re-apply the script if you changed the path of your nulls.
If I knew a little more about your project I may be able to suggest some other options.