Vlv2TbltDevicePkg: Fix typo in name of nul output file

Vlv2TbltDevicePkg: Fix typo in name of nul output file

https://bugzilla.tianocore.org/show_bug.cgi?id=272

Fix typo in script file.  To prevent output from being
shown, then output file should be 'nul', not 'null'.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: David Wei <david.wei@intel.com>
Cc: Mang Guo <mang.guo@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Michael Kinney <michael.d.kinney@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>