[cleanup] Drop empty `class` attribute from `#footer > ul` elements

Change-Id: Ice0241b054cbc296e9f0adf1d2920023316c8ca0

includes/VectorTemplate.php

@@ -243,7 +243,7 @@ class VectorTemplate extends BaseTemplate {
 
 			$footerRows[] = [
 				'id' => $rowId,
-				'className' => '',
+				'className' => null,
 				'array-items' => $items
 			];
 		}

includes/templates/Footer.mustache

@@ -16,7 +16,7 @@
 {{{html-hook-vector-before-footer}}}
 <div id="footer" class="mw-footer" role="contentinfo" {{{html-userlangattributes}}}>
 	{{#array-footer-rows}}
-	<ul id="{{id}}" class="{{className}}">
+	<ul id="{{id}}" {{#className}}class="{{.}}"{{/className}}>
 		{{#array-items}}
 		<li id="{{id}}">{{{html}}}</li>
 		{{/array-items}}