Add args checks and style consistency nits

[siv2r]

This PR adds:

• scalar checks to schnorr_extract_adaptor and schnorr_adapt functions.
• uses is None to check for infinity point instead of calling is_infinity func

  • this style is consistent with the bip340 reference code

Question:

• Should we remove T0 is None check (at the end) in the extract_t func?
  • since there is T0 is None check is present in schnorr_pre_verifty.
  • also nothing wrong when returning a None point (like point_add does).
  • but again infinity is an invalid adaptor point. So, maybe we shouldn't remove this check.