BaseTools/GenFfs: Fix return too early when input file is of size 0

BaseTools/GenFfs: Fix return too early when input file is of size 0

Commit 2cb874352423fcfd180199e6de8298567dff8e7f eliminates possible NULL
pointer dereference in GenFfs tool source codes. However, it doesn't
correctly handle the case when the input file is of size 0. This will lead
to possible build issues.

This commits refine the logic to handle the above case.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>