cmState: Only append non-empty values to buildsystem properties.

This is a change in behavior from CMake 3.3, but there is no semantic meaning
to empty entries in buildsystem properties.  This also restores behavior to
that of CMake 2.8.10.

Source/cmState.cxx

@@ -1378,6 +1378,11 @@ template <typename T, typename U, typename V>
 void AppendEntry(T& content, U& backtraces, V& endContentPosition,
     const std::string& value, const cmListFileBacktrace& lfbt)
 {
+  if (value.empty())
+    {
+    return;
+    }
+
   assert(endContentPosition == content.size());
 
   content.push_back(value);