crypto: inside-secure - add support for 0 length HMAC messages

This patch adds support for the specific corner case of performing HMAC
on an empty string (i.e. payload length is zero). This solves the last
failing cryptomgr extratests for HMAC.

Signed-off-by: Pascal van Leeuwen <pvanleeuwen@verimatrix.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>