Don't make query if not needed (#2863)

* Don't make query if not needed

See silverstripe/silverstripe-assets#557 for background

* add comment

code/Model/SiteTreeLinkTracking.php

@@ -116,15 +116,22 @@ public function augmentSyncLinkTracking()
         $allFields = DataObject::getSchema()->fieldSpecs($this->owner);
         $linkedPages = [];
         $anyBroken = false;
+        $hasTrackedFields = false;
         foreach ($allFields as $field => $fieldSpec) {
             $fieldObj = $this->owner->dbObject($field);
             if ($fieldObj instanceof DBHTMLText) {
+                $hasTrackedFields = true;
                 // Merge links in this field with global list.
                 $linksInField = $this->trackLinksInField($field, $anyBroken);
                 $linkedPages = array_merge($linkedPages, $linksInField);
             }
         }
 
+        // We need a boolean flag instead of checking linkedPages because it can be empty when pages are removed
+        if (!$hasTrackedFields) {
+            return;
+        }
+
         // Soft support for HasBrokenLink db field (e.g. SiteTree)
         if ($this->owner->hasField('HasBrokenLink')) {
             $this->owner->HasBrokenLink = $anyBroken;