There's a number of different ways to do this and here's a couple:
i) Use a Beam effect, and set one end to your static point. Then use an expression and pickwhip the Null's position to the second point.
ii) Use a lovely "Point At" expression on a line shape. Apply this to the rotation of you line layer:
from=transform.position;
to=thisComp.layer("YOUR LAYER").transform.position;
delta=sub(from,to);
angle=Math.atan2(delta[1],delta[0]);
radiansToDegrees(angle)+180;
Just change the layer name in line 2 to your Null layer's name.
All the best,
Ben
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