I am aware that obligatory expletives did not exist in early ON and perhaps also not in early OHG, but my knowledge of the specifics is hazy. In OE at least, I believe expletives in conjunction with weather-verbs dates back as far as we have attestations of weather verbs.
I lean to the hypothesis that there is no evidence for them, or that if there is, it is confined to specific semantic types of verb. I was having a discussion with someone about whether it makes sense to posit an etymological or null (which imo are very different things) dummy subject in oblique subject constructions; to me, this seems to add unnecessary assumptions given both typological and IE-internal evidence. Any further reading relevant to this corollary is welcome.