FIX Add an empty string to the subsite operations dropdown.

This prevents a subsite from being chosen as the default option, assuming this is undesired

src/Extensions/SiteTreeSubsites.php

@@ -124,7 +124,8 @@ public function updateCMSFields(FieldList $fields)
                         DropdownField::create('CopyToSubsiteID', _t(
                             __CLASS__ . '.CopyToSubsite',
                             'Copy page to subsite'
-                        ), $subsitesMap),
+                        ), $subsitesMap)
+                        ->setEmptyString('Select an Option'),
                         CheckboxField::create(
                             'CopyToSubsiteWithChildren',
                             _t(__CLASS__ . '.CopyToSubsiteWithChildren', 'Include children pages?')