When rows are sorted using an ORDER BY clause, nulls sort as the lowest value.
If any row has a null in the column being grouped, then all rows having a null are placed into one group. In other words, though it is syntactically incorrect to formulate the predicate NULL = NULL in a DML request because nulls have no value and therefore cannot be equated, it is also true that when SQL sorts nulls, the implicit result is that NULL = NULL.