Fix 'Unable to read existing migrations' error on PHP 5.2.5 or later.

scafold : Error: Unable to read existing migrations. Do you have an 'open_basedir' defined?
http://fuelphp.com/forums/discussion/10303/scafold-error-unable-to-read-existing-migrations-do-you-have-an-039open_basedir039-defined

glob() returns FALSE instead of an empty array since PHP 5.2.5 when wildcard is used in the argument and  the directory is empty.

Backward Incompatible Changes
http://www.php.net/manual/en/migration52.incompatible.php

Signed-off-by: Hideki Okamoto <tox2ro@gmail.com>