crypto: return NULL cryptoctx if protocol string is empty.

crypto: return NULL cryptoctx if protocol string is empty.

Badly formed messages that don't specify a protocol in
signed/encrypted parts, end up with a protocol of NULL.  strcasecmp in
notmuch_crypto_get_context then segfaults when trying to check it
against known protocols.  If the protocol is NULL, just return an
empty context immediately (with appropriate message.)