Try this.
<h3> <%= typeof page_title !== 'undefined' ? page_title : '' %> </h3>
But that will still render the h3 tag if page_title is not present in the route.
Or you could try this if you don’t want to render the h3 tag at all if page_title is not present
<%if (typeof page_title !== 'undefined') { %>
<h3> <%= page_title %> </h3>
<% } %>
For reference Wappler uses EJS templating with Express for nodejs. So if you want to dig into these options you can have a look at.