What is an external nulling "resistor"?
Take a look at a randomly chosen old op-amp, for example the TL081. The single-amp version will often have two pins named "offset null" or similar.

This is because they have a fairly large offset voltage that can cause problems if you want to amplify DC levels, and the offset voltage is different for each IC.

There are many ways to connect these pins but normally it's just a trimpot connected to the pins, with the wiper connected to the most negative supply rail.