so ~ that 구문에서 that 생략

He was a thin leathery man with colorless eyes, so colorless (that) they did not reflect light.
to kill a mockingbird, harper lee

눈이 너무 퀭해서 암흑 같았다는 내용이다. 저렇게 접속사 that을 생략할 수는 있는데 제대로 된 문장에서는 그러지 않는다.

When the conjunction that is part of the correlative pairs so … that, such … that, now … that (or so that, etc.), it is normally retained in formal writing but is sometimes omitted in informal contexts …
fowler’s dictionary of modern english usage, jeremy butterfield