Fix system warning in gpc_get_string_array()

The fix for issue #17640 did not consider that the value returned by
gpc_get() is not necessarily an array - it can be the default value
(e.g. null) causing PHP to throw an 'Invalid argument supplied for
foreach()' warning.

Fixes #17967, regression from 215968fa8ff33e327f0600765a5caa24de392cbc
Fix #17977: handling of due dates

Commit 215968fa8ff33e327f0600765a5caa24de392cbc (backported from master
fc02c46eea9d9e7cc472a7fc1801ea65d467db76 to fix issue #17640) added
stripping of null bytes in GPC API, but did not correctly handle null
values.

This is a backport of commit f725b46954a514880792dd4be8228287756fac3d
from master branch, to address this issue.

Signed-off-by: Damien Regad <dregad@mantisbt.org>