I haven't completed the Alola Pokedex in Pokemon Sun so haven't got the shiny charm but would like the shiny Type:Null. Is it still possible to find it with soft resetting or do I need the shiny charm?
You can find any shiny without the shiny charm, unless it is shiny locked, and Type:Null is not shiny-locked, so start soft reseting!
Source: watching people shiny hunt it
It is possible to shiny hunt any Pokemon in any game even without a Shiny Charm. I have done it many times.
Source: Resetting for Shiny Rowlet in Ultra Moon, and Shiny Fennekin in Y