This is a contribution to help others who might find this confusing:

In French, there are certain sentence constructions or even verbs that could be made negative with just the addition of "ne". The addition of "pas" and the likes isn't necessary. This concept is called "Ne littéraire". 

Clauses containing "si" are one of those instances where the Ne littéraire could be used.

For further study of the concept, check: https://www.lawlessfrench.com/grammar/ne-litteraire/