fix(gatsby): fix codepaths that result in js errors (#21829)

Author: wardpeet

packages/gatsby/cache-dir/loader.js

@@ -353,7 +353,9 @@ const createComponentUrls = componentChunkName =>
 export class ProdLoader extends BaseLoader {
   constructor(asyncRequires, matchPaths) {
     const loadComponent = chunkName =>
-      asyncRequires.components[chunkName]().then(preferDefault)
+      asyncRequires.components[chunkName]
+        ? asyncRequires.components[chunkName]().then(preferDefault)
+        : Promise.resolve()
 
     super(loadComponent, matchPaths)
   }

packages/gatsby/cache-dir/navigation.js

@@ -84,7 +84,10 @@ const navigate = (to, options = {}) => {
     if (!pageResources || pageResources.status === `error`) {
       window.history.replaceState({}, ``, location.href)
       window.location = pathname
+      clearTimeout(timeoutId)
+      return
     }
+
     // If the loaded page has a different compilation hash to the
     // window, then a rebuild has occurred on the server. Reload.
     if (process.env.NODE_ENV === `production` && pageResources) {