Fix table variable PHP notice

Author: avonville

includes/integrations/sprout-invoices/class-integration-sprout-invoices.php

@@ -277,6 +277,7 @@ public static function build_entry_note($form_id, $entry_id)
             return '';
         }
 
+        $table = ' ';
         $table .= '<div class="submission-values">';
         $table .= '<ul>';