This question has been marked as a duplicate of a question about so-called weather-it. This question here has nothing to do with weather it. It seems from one of the answers that it might be from an extraposition construction, or from a referential usage. This has nothing to do with 'weather-it' at all.
Is the word 'It' in:
- It's a pity
a pronoun? Or does it just serve as a filler because otherwise there isn't a subject in the sentence?
Is the situation different in the sentence:
- It's a pity you cannot come.