Only print page info when there is date available

Useful for sub-pages which do not necessarily have such metadata.

templates/macros.html

@@ -17,10 +17,12 @@
 {% endmacro %}
 
 {% macro page_info(page) %}
+	{% if page.date %}
 		<div class="info">
 			<span class="date">{{ page.date }}</span>
 			{%- if page.extra.author -%}
 				, <span class="author">{{ page.extra.author }}</span>
 			{% endif %}
 		</div>
+	{% endif %}
 {% endmacro %}