added the empty test

doc/templates.rst

@@ -1408,6 +1408,18 @@ useful if you use the ``strict_variables`` option:
         ...
     {% endif %}
 
+``empty``
+~~~~~~~~~
+
+``empty`` checks if a variable is empty:
+
+.. code-block:: jinja
+
+    {# evaluates to true if the foo variable is null, false, or the empty string #}
+    {% if foo is empty %}
+        ...
+    {% endif %}
+
 Extensions
 ----------