Fix bug whereby Series.diff 1 on empty series is an error
Fix bug whereby Series.diff 1 on empty series is an error

jeyoor/Deedle,BlueMountainCapital/Deedle,filmor/Deedle,buybackoff/Deedle,BlueMountainCapital/Deedle,Dependencies/Deedle,jeyoor/Deedle,caioproiete/Deedle,filmor/Deedle,AtwooTM/Deedle,BlueMountainCapital/Deedle,tpetricek/Deedle,tpetricek/Deedle,AtwooTM/Deedle,buybackoff/Deedle,Dependencies/Deedle,tpetricek/Deedle,buybackoff/Deedle,filmor/Deedle,AtwooTM/Deedle,caioproiete/Deedle,caioproiete/Deedle,jeyoor/Deedle,Dependencies/Deedle