MdeModulePkg/Variable: Initialize local variable "Variable"

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=2364

Fixes a new build warning in VS2012 introduced in f8ff4cca7c.

This patch initializes the local variable "Variable" in
VariableServiceGetNextVariableInternal ().

This ensures the pointers in the structure are initialized
in the case no variable stores exist in the list of variable
stores.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Hao A Wu <hao.a.wu@intel.com>
Signed-off-by: Michael Kubacki <michael.a.kubacki@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Reviewed-by: Jian J Wang <jian.j.wang@intel.com>