Remove nonnull warning when calling secp256k1_schnorr_sign with NULL noncefp

Summary: This warning was hidden under normal conditions because SECP256K1_ARG_NONNULL is bypassed when SECP256K1_BUILD is set. A NULL noncefp means the default nonce function is used.

Test Plan: make check

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Subscribers: schancel

Differential Revision: https://reviews.bitcoinabc.org/D2747