BaseTools/GenFds: Fix 'NoneType' object is not iterable error.

BaseTools/GenFds: Fix 'NoneType' object is not iterable error.

When adding section VERSION in FDF file, for example:
FILE FREEFORM = PCD(gEfiIntelFrameworkModulePkgTokenSpaceGuid.PcdLogoFile) {
    SECTION RAW = MdeModulePkg/Logo/Logo.bmp
    SECTION UI = "Logo"
    SECTION VERSION = "0001"
  }
GenFds will report the following error:
Traceback (most recent call last):
  File "GenFds.py", line 276, in main
  File "GenFds.py", line 391, in GenFd
  File "Fd.py", line 93, in GenFd
  File "Region.py", line 106, in AddToBuffer
  File "Fv.py", line 114, in AddToBuffer
  File "FfsFileStatement.py", line 117, in GenFfs
  File "VerSection.py", line 80, in GenSection
  File "GenFdsGlobalVariable.py", line 401, in GenerateSection
TypeError: 'NoneType' object is not iterable.
We found in GenFdsGlobalVariable.py line 401 'list' requires a iteralbe object as parameter while the 'Input' is None.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Qiu Shumin <shumin.qiu@intel.com>
Reviewed-by: Yingke Liu <yingke.d.liu@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18205 6f19259b-4bc3-4df7-8a09-765794883524