fix: 1c09422 error calling isset

error calling isset: calling IsSet with unsupported type "invalid" (<nil>) will always return false

See https://github.com/wowchemy/wowchemy-hugo-themes/commit/1c094227ecea2f534321f80c6113522b234e5012#commitcomment-123412561

modules/wowchemy/layouts/partials/site_js.html

@@ -151,13 +151,13 @@
 {{ $use_headroom := not (.IsHome | or (eq .Type "book")) }}
 
 {{/* Check for the global site parameter `header.on_scroll` */}}
-{{ if (isset site.Params.header "on_scroll") }}
+{{ if site.Params.header.on_scroll }}
   {{ $use_headroom = eq site.Params.header.on_scroll "disappear" }}
 {{ end }}
 
 {{/* Check for the page parameter `header.on_scroll` */}}
-{{ if (isset $.Params.header "on_scroll") }}
-  {{ $use_headroom = eq $.Params.header.on_scroll "disappear" }}
+{{ if .Params.header.on_scroll }}
+  {{ $use_headroom = eq .Params.header.on_scroll "disappear" }}
 {{ end }}
 
 {{/* Output `use_headroom` variable for Headroom initialization in Wowchemy JS */}}