Answered Output Filter - If 1st item is empty, then modify 2nd item

<ul class="nav nav-tabs">
	<li class="active"><a href="#item1" data-toggle="tab">Item 1</a></li>
	[[*item2:!empty=`<li><a href="#item2" data-toggle="tab">Item 2</a></li>`]]
	[[*item3:!empty=`<li><a href="#item3" data-toggle="tab">Item 3</a></li>`]]
	[[*item4:!empty=`<li><a href="#item4" data-toggle="tab">Item 4</a></li>`]]
	[[*item5:!empty=`<li><a href="#item5" data-toggle="tab">Item 5</a></li>`]]
	[[*item6:!empty=`<li><a href="#item6" data-toggle="tab">Item 6</a></li>`]]
	[[*item7:!empty=`<li><a href="#item7" data-toggle="tab">Item 7</a></li>`]]
</ul>

<?php
$output = '';
$tvIds = '1,77,9,12';
$tvIdArray = explode(',', $tvIds);

$class = ' class="active" '; // Don't forget the spaces at each end
$i = 1;
foreach ($tvIdArray as $k => $tvId) {
    $tv = $modx->resource->getTVValue( (integer) $tvId);
    if (!empty($tv)) {
        $output .= '<li' . $class . '><a href="#item' . $i . '" data-toggle="tab">Item ' . $i . '</a></li>';         
        $class = '';
        $i++;
    }
}
return $output;