Null liquid does absolutely nothing. No reactions. It just flows. Null, dull, and boring. There are two keys for Null: A and N. When you select the material with those keys (and not pressing escape) the two materials will have (almost) exactly the same color.
When you use the Mix Mode to make A-type Null, it is paler than normal. N-type Null stays the same. Other materials with A in will become paler as well. Inflow with A in will give paler materials.
You can mix walls with both types of null to form a null wall, but it does not share the non-reactive properties of null. Null is useful because it does not react with Snow.
The word Null is another way of saying the word nothing.