ensure plugin object is defined when dereferencing payloadHash
ensure plugin object is defined when dereferencing payloadHash

It is possible (though not valid) for a request to contain
a hash in the Authorization header but no payload in the
request body. When this happens plugins['hapi-auth-hawk']
will not be defined, so dereferencing payloadHash throws
an error. This patch ensure payloadHash can always be
dereferenced.

lloydbenson/hapi-auth-hawk