effectiveDate might be falsy (#23002)

Part of #1185

Author: Peter Bengtsson

middleware/render-page.js

@@ -34,7 +34,10 @@ export default async function renderPage(req, res, next) {
 
   // Updating the Last-Modified header for substantive changes on a page for engineering
   // Docs Engineering Issue #945
-  if (context.page.effectiveDate !== '') {
+  if (context.page.effectiveDate) {
+    // Note that if a page has an invalidate `effectiveDate` string value,
+    // it would be caught prior to this usage and ultimately lead to
+    // 500 error.
     res.setHeader('Last-Modified', new Date(context.page.effectiveDate).toUTCString())
   }