cmCxxModuleMetadata: Don't serialize default values

Serializing defaults inside each preprocessor define inflats manifest
size and makes auditing them more difficult. When "value" is empty
or "undef" is false, don't serialize them at all.

Source/cmCxxModuleMetadata.cxx

@@ -300,10 +300,10 @@ Json::Value SerializePreprocessorDefine(
   dv["name"] = d.Name;
   if (d.Value) {
     dv["value"] = *d.Value;
-  } else {
-    dv["value"] = Json::Value::null;
   }
-  dv["undef"] = d.Undef;
+  if (d.Undef) {
+    dv["undef"] = d.Undef;
+  }
   return dv;
 }