This reverts commit 3b8459d7.

Return NULL and report errors in the log if a HMAC key or message
is NULL, rather than tripping assertions.

we would need to complicate testing specifically, so just mark this as
a TODO for now

object is NULL. If the blob's length is 0 or its priv pointer is NULL,
pass over it silently -- not an error, but no need to call the hash
update.

allow the .final() method to be called more than once, caching the result
on the first call