Fix reading NULL Big Int data types.

[fabianosim]

There is a bug when you have a NULL column from Big Int data type. The code throws the message "Reading while at end of stream" and gives a PHP Notice. To fix this, the protected function "read()" must check if the length of the data being read returns false. If the column is NULL, then $length will be 0 and the error will be thrown. This commit fixes this issue.