Fix default port to null for hhvm

HHVM doesn't allow false to be passed as port to mysqli_real_connect.
It only allows a nullable integer until ZendParamMode is finished.

See facebook/hhvm#1817

Signed-off-by: Tim Siebels <tim_siebels_aurich@yahoo.de>
Merge pull request #1299 from SiebelsTim/fix-hhvm-serverport

Fix default port to null for hhvm