Is there, like, a word for what it is doing in English constructions like "it's raining" or "it's hot"?

Normally it is a pronoun which stands in for a noun that has been previously referred to, or is obvious from context. But what is it standing in for in these sentences? In English, "raining" is a verb that actually does not take a subject. Sort of like the opposite of an intransitive verb. It can rain frogs. It can rain blood. But nothing can rain.

Like, replace It with any conceivable word that might be being referred to in the sentence "It's raining." None of those replacements will be correct.

Is there a linguistic term for this phenomenon of an "empty" pronoun being used to stand in for the lack of a subject?
Also, is there any established term for a verb that takes no subject? The closest I can think of is ergative but I'm not sure that's quite accurate.