Fix compile error on Apple Clang

Needs to compile with `CMAKE_OSX_DEPLOYMENT_TARGET` of 10.14 at least to
have `std::optional<>::value()` available. So we have to increase the
minimum required compiler version, but that requires more changes. Just
fix the build error for now.

source/blender/blenlib/intern/dot_export.cc

@@ -237,7 +237,7 @@ void NodePort::to_dot_string(std::stringstream &ss) const
 {
   m_node->export__as_id(ss);
   if (m_port_name.has_value()) {
-    ss << ":" << m_port_name.value();
+    ss << ":" << *m_port_name;
   }
 }