Thats so nice of you là gì

What is the grammatically correct sentence?

1. That's nice of you to say so!
2. It's nice of you to say so!

I've come across 1 many times, but I feel it seems not to be grammatically correct, strictly speaking as "that" cannot be used as a dummy pronoun, so it should be written as 2 as "it" can work as a dummy pronoun referring to "to say so". If you think of 1 as natural, could you tell me why?

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