do not clean up whitespace text nodes inside if tags

src/Node/IfNode.php

@@ -50,8 +50,11 @@ class IfNode extends Node
                 ->subcompile($this->getNode('tests')->getNode($i))
                 ->raw(") {\n")
                 ->indent()
-                ->subcompile($this->getNode('tests')->getNode($i + 1))
             ;
+            // The node might not exists if the content is empty
+            if ($this->getNode('tests')->hasNode($i + 1)) {
+                $compiler->subcompile($this->getNode('tests')->getNode($i + 1));
+            }
         }
 
         if ($this->hasNode('else')) {

tests/Fixtures/tags/if/empty_body.test

@@ -0,0 +1,32 @@
+--TEST--
+empty "if" body in child template
+--TEMPLATE--
+{% extends 'base.twig' %}
+
+{% set foo = '' %}
+
+{% if a is defined %}
+
+{% else %}
+    {% set foo = 'NOTHING' %}
+{% endif %}
+
+{% if a is defined %}
+
+{% endif %}
+
+{% if a is defined %}
+    {% set foo = 'NOTHING' %}
+{% else %}
+
+{% endif %}
+
+{% block content %}
+    {{ foo }}
+{% endblock %}
+--TEMPLATE(base.twig)--
+{% block content %}{% endblock %}
+--DATA--
+return []
+--EXPECT--
+  NOTHING