MapFields: fix mapfield_instructions PHP notice

templates/CRM/Advimport/Upload/Form/MapFields.tpl

@@ -16,7 +16,7 @@
           <div class="messages help">
             <p><i class="fa fa-check-circle fa-2x"></i> {ts}The data looks OK, you can continue to the next step.{/ts}</p>
           </div>
-        {elseif $mapfield_instructions}
+        {elseif !empty($mapfield_instructions)}
           <div class="messages help">
             <p>{$mapfield_instructions}</p>
           </div>