Because there is no initial ng sound in modern Korean language, the textbook said there is no liaison after ㅇ before vowel.
But, in ancient Korean language, there was initial ng sound called 옛이응.
You can find the trace in 아지(baby) of 강아지,송아지,망아지.
개+아지=>강아지
소+아지=>송아지
말+아지=>망아지
Officially, there is no liaison, but actually there is a liaison of 옛이응.