BaseTools: Fix a bug for Unused PCDs section display in the report

BaseTools: Fix a bug for Unused PCDs section display in the report

Fix a regression issue caused by ac4578af364, when there doesn't exist
not used PCD, it also display the not used Pcd section in the report.

Fixes: https://bugzilla.tianocore.org/show_bug.cgi?id=1170
Cc: Liming Gao <liming.gao@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Zhiju.Fan <zhijux.fan@intel.com>
Reviewed-by: Yonghong Zhu <yonghong.zhu@intel.com>