test: add test case for empty data gram packets
udp: fix send so it accepts buffers with length 0
test: add test for sending empty buffers with udp.send
udp: pass the length of the sent size to the callback

There is no value in passing buffer.length, the user can
check the buffer length himself. The user should be more
interested in the length of the sent content.
test: add test for length udp send callback value
Revert "udp: pass the length of the sent size to the callback"

This reverts commit 4d0d0fa8fd8a32b2cac68899d1af25a0cc6d3cc0.
lib/dgram.js use trevnorris method to pass length