minor #1687 Update empty.rst (violuke)

This PR was merged into the 2.0-dev branch.

Discussion
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Update empty.rst

Correcting English.

Commits
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d2d7ec8 Update empty.rst

doc/tests/empty.rst

@@ -5,7 +5,7 @@
 
 .. code-block:: jinja
 
-    {# evaluates to true if the foo variable is null, false, an empty array, or the empty string #}
+    {# evaluates to true if the foo variable is null, false, an empty array, or an empty string #}
     {% if foo is empty %}
         ...
     {% endif %}