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

Fix a regression issue caused by ac4578af36, 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>

BaseTools/Source/Python/build/BuildReport.py

@@ -888,7 +888,17 @@ class PcdReport(object):
             if self.ConditionalPcds:
                 self.GenerateReportDetail(File, ModulePcdSet, 1)
             if self.UnusedPcds:
-                self.GenerateReportDetail(File, ModulePcdSet, 2)
+                IsEmpty = True
+                for Token in self.UnusedPcds:
+                    TokenDict = self.UnusedPcds[Token]
+                    for Type in TokenDict:
+                        if TokenDict[Type]:
+                            IsEmpty = False
+                            break
+                    if not IsEmpty:
+                        break
+                if not IsEmpty:
+                    self.GenerateReportDetail(File, ModulePcdSet, 2)
         self.GenerateReportDetail(File, ModulePcdSet)
 
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