gspawn, gdataset: Restore nullable callback functions

As per commit 5d738ddc some callbacks are not nullable anymore even though they should be.

This breaks the introspection as some arguments won't be considered nullable anymore.

You can notice this in https://gitlab.gnome.org/3v1n0/gjs/-/pipelines/558839 where some some tests are running against stable g-i version and others (the failing ones) against the devel one.

/cc @mcatanzaro: it seems that !3287 (comment 1795889) wasn't addressed completely, wasn't it?

I didn't change them all, as some removal on GFile still make sense.