Passing pointers to ZST to FFI

[elichai]

We have a bunch of functions that accept slices, these slices could be Zero Sized types if they're empty.
getting a pointer to ZST is implementation defined and could be NULL which might violates the ffi contract for non null values.
We should either explicitly check that the slices aren't empty or have our own pointer trait that always return a valid pointer

https://doc.rust-lang.org/nomicon/exotic-sizes.html#zero-sized-types-zsts

Example of having "our own as_ptr" https://github.com/enigmampc/enigma-core/blob/develop/enigma-types/src/traits.rs

I personally lean towards the first option of explicitly checking, the only problem with that is that we might forget. then it will hopefully call secp256k1_callback_call in the good case, and in a bad case create a UB

[apoelstra]

If we get UB that is a bug upstream. No such bugs should exist.

[elichai]

Well upstream docs do specify Cannot be NULL sometimes, I agree it should be checked there and I'm pretty sure it is checked everywhere, but the docs don't explicitly say that.

[apoelstra]

Every single cannot be NULL should be accompanied by an ARG_CHECK asserting that the argument is non-NULL. I'm not aware of any exceptions.

[elichai]

Solved by #151