MdeModulePkg:Fix the issue that Commit changes and Exit fail when add driver option

MdeModulePkg:Fix the issue that Commit changes and Exit fail when add driver option

When add driver option using file,input the description and then commit changes and exit,
it doesn't work.it caused by the commit 18216.When variable DriverOrderList in Variable.c
is NULL,it also need do the follow path,shouldn't return.Now change the code.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Qiu Shumin <shumin.qiu@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18220 6f19259b-4bc3-4df7-8a09-765794883524