Fix undefined path (#621)

* Fix undefined path

Author: Josep Martins

.changeset/lovely-experts-watch.md

@@ -0,0 +1,5 @@
+---
+'@primer/gatsby-theme-doctocat': patch
+---
+
+Fixes selected state when the path is `undefined`

theme/src/components/header.js

@@ -156,7 +156,7 @@ function PrimerNavItems({siteMetadata, items, path, pathPrefix}) {
             <UnderlineNav.Link
               key={index}
               href={item.url}
-              selected={item.url === siteMetadata.header.url + pathPrefix + path}
+              selected={item.url === siteMetadata.header.url + (pathPrefix || '') + (path || '')}
               sx={{fontSize: 2, lineHeight: 'condensed'}}
             >
               {item.title}