Fix 500 when saving /site/edit without email
When saving /site/edit with no no_reply_email set, a 500 error page
would be returned due to an empty value being passed as value to
the datastore (which is not allowed for email fields).
<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
{% spaceless %}
{% for url in urlset %}
<url>
<loc>{{ url.location }}</loc>
{% if url.lastmod %}<lastmod>{{ url.lastmod|date:"Y-m-d" }}</lastmod>{% endif %}
{% if url.changefreq %}<changefreq>{{ url.changefreq }}</changefreq>{% endif %}
{% if url.priority %}<priority>{{ url.priority }}</priority>{% endif %}
</url>
{% endfor %}
{% endspaceless %}
</urlset>