ShellPkg: update behavior with undefined environment variables
ShellPkg: update behavior with undefined environment variables

Undefined environment variables are now removed during script execution.
Excepted environment variables are now correctly un-excepted right before processing continues

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jaben Carsey <Jaben.carsey@intel.com>
reviewed-by: Matthews, Robert <Robert.Matthews@hp.com>

git-svn-id: 3158a46dfd52e07d1fda3e32e1ab2e353a00b20f@14585 6f19259b-4bc3-4df7-8a09-765794883524

MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2