FIX Allow empty PreviewURLs for CMSPreviewable objects

An empty PreviewURL will result in the "no preview available" message
displaying instead of a 404 error.

code/Controllers/SilverStripeNavigatorItem_ArchiveLink.php

@@ -46,10 +46,8 @@ class SilverStripeNavigatorItem_ArchiveLink extends SilverStripeNavigatorItem
 
     public function getLink()
     {
-        return Controller::join_links(
-            $this->record->PreviewLink(),
-            '?archiveDate=' . urlencode($this->record->LastEdited ?? '')
-        );
+        $link = $this->record->PreviewLink();
+        return $link ? Controller::join_links($link, '?archiveDate=' . urlencode($this->record->LastEdited ?? '')) : '';
     }
 
     public function canView($member = null)
@@ -62,6 +60,7 @@ class SilverStripeNavigatorItem_ArchiveLink extends SilverStripeNavigatorItem
             && $this->isArchived()
             // Don't follow redirects in preview, they break the CMS editing form
             && !($record instanceof RedirectorPage)
+            && $this->getLink()
         );
     }
 

code/Controllers/SilverStripeNavigatorItem_LiveLink.php

@@ -48,7 +48,8 @@ class SilverStripeNavigatorItem_LiveLink extends SilverStripeNavigatorItem
 
     public function getLink()
     {
-        return Controller::join_links($this->getLivePage()->PreviewLink(), '?stage=Live');
+        $link = $this->getLivePage()->PreviewLink();
+        return $link ? Controller::join_links($link, '?stage=Live') : '';
     }
 
     public function canView($member = null)
@@ -60,6 +61,7 @@ class SilverStripeNavigatorItem_LiveLink extends SilverStripeNavigatorItem
             && $this->showLiveLink()
             && $record->hasStages()
             && $this->getLivePage()
+            && $this->getLink()
         );
     }
 

code/Controllers/SilverStripeNavigatorItem_StageLink.php

@@ -50,8 +50,12 @@ class SilverStripeNavigatorItem_StageLink extends SilverStripeNavigatorItem
     public function getLink()
     {
         $date = Versioned::current_archived_date();
+        $link = $this->record->PreviewLink();
+        if (!$link) {
+            return '';
+        }
         return Controller::join_links(
-            $this->record->PreviewLink(),
+            $link,
             '?stage=Stage',
             $date ? '?archiveDate=' . $date : null
         );
@@ -66,6 +70,7 @@ class SilverStripeNavigatorItem_StageLink extends SilverStripeNavigatorItem
             && $this->showStageLink()
             && $record->hasStages()
             && $this->getDraftPage()
+            && $this->getLink()
         );
     }
 

code/Controllers/SilverStripeNavigatorItem_Unversioned.php

@@ -19,7 +19,7 @@ class SilverStripeNavigatorItem_Unversioned extends SilverStripeNavigatorItem
 
     public function getLink()
     {
-        return $this->getRecord()->PreviewLink();
+        return $this->getRecord()->PreviewLink() ?? '';
     }
 
     public function getTitle()
@@ -42,6 +42,7 @@ class SilverStripeNavigatorItem_Unversioned extends SilverStripeNavigatorItem
         return (
             !$this->getRecord()->hasExtension(Versioned::class)
             && $this->showUnversionedLink()
+            && $this->getLink()
         );
     }