core: Allow finalization of signature verification of unencrypted data

* src/decrypt-verify.c (decrypt_verify_status_handler): Call
_gpgme_verify_status_handler on EOF even if
_gpgme_decrypt_status_handler returned NO DATA error.
* tests/gpg/t-decrypt-verify.c (normal_signed_message): New.
(main): Add test with signed, but not encrypted data.

This allows the verify status handler to finalize the verification of
the last signature even if the decrypt status handler returned a NO DATA
error because the input data wasn't encrypted.

• GnuPG-bug-id: T6368