REST API: JS Client - improve collection route construction for empty parents.

Fix an issue where the constructed path for hierarchical collections could contain a double slash ("//") when items contained empty parents, causing an error.

Props nicomollet.
Fixes #44745.


git-svn-id: https://develop.svn.wordpress.org/trunk@49390 602fd350-edb4-49c9-b593-d223f7449a82

src/js/_enqueues/wp/api.js

@@ -1412,7 +1412,10 @@
 						// Function that returns a constructed url passed on the parent.
 						url: function() {
 							return routeModel.get( 'apiRoot' ) + routeModel.get( 'versionString' ) +
-									parentName + '/' + this.parent + '/' +
+								parentName + '/' +
+								( ( _.isUndefined( this.parent ) || '' === this.parent ) ?
+									( _.isUndefined( this.get( 'parent_post' ) ) ? '' : this.get( 'parent_post' ) + '/' ) :
+									this.parent + '/' ) +
 								routeName;
 						},