test: remove common.PORT from dgram test

Remove common.PORT from
test-dgram-send-callback-buffer-length-empty-address to eliminate
possibility that a dynamic port used in another test will collide
with common.PORT.

PR-URL: https://github.com/nodejs/node/pull/12944
Refs: https://github.com/nodejs/node/issues/12376


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <mhdawson@ibm.com>