Fix crypto.hmac behavior with empty keys.

node 0.9.6 introduced Buffer changes that cause the key argument of
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.