Fix crypto.hmac behavior with empty keys.

node 0.9.6 introduced Buffer changes that cause the key argument of
Hmac::HmacInit (used in crypto.createHmac) to be NULL when the key is
empty. This argument is passed to OpenSSL's HMAC_Init, which does not
like NULL keys.

This change works around the issue by passing an empty string to
HMAC_Init when the key is empty, and adds crypto.createHmac tests for
the edge cases of empty keys and values.