ShellPkg: replace unrecognized Environment Variables with empty quotes so commands or applications know something existed.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jaben Carsey <Jaben.carsey@intel.com>
Reviewed-by: Tapan Shah <tapandshah@hp.com>
Reviewed-by: Erik Bjorge <erik.c.bjorge@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15547 6f19259b-4bc3-4df7-8a09-765794883524