routing: check to see if pubkey is nil before logging in sendPayment
routing: check to see if pubkey is nil before logging in sendPayment

In this commit, we now need to check to see if the pubkey is nil as if
the user specified a set of manual routes, then the dest pubkey would be
empty.

aakselrod/lnd,aakselrod/lnd,LightningNetwork/lnd,LightningNetwork/lnd