{% if app['google.analytics.id'] %}
    <script type="text/javascript">
        var _gaq = _gaq || [];
        _gaq.push(['_setAccount', '{{ app["google.analytics.id"] }}']);
        _gaq.push(['_trackPageview']);

        (function(){
            var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true;
            ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js';
            var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s);
        })();
    </script>
{% endif %}

{% if app['google.analytics.id']|default(false) %}
    <script type="text/javascript">
        var _gaq = _gaq || [];
        _gaq.push(['_setAccount', '{{ app["google.analytics.id"] }}']);
        _gaq.push(['_trackPageview']);

        (function(){
            var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true;
            ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js';
            var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s);
        })();
    </script>
{% endif %}

Check for Google Analytics key with isset()
Check for Google Analytics key with isset()