do not clean up whitespace text nodes inside if tags

src/Parser.php

@@ -17,6 +17,7 @@
 use Twig\Node\BlockReferenceNode;
 use Twig\Node\BodyNode;
 use Twig\Node\Expression\AbstractExpression;
+use Twig\Node\IfNode;
 use Twig\Node\MacroNode;
 use Twig\Node\ModuleNode;
 use Twig\Node\Node;
@@ -381,6 +382,10 @@ private function filterBodyNodes(Node $node, bool $nested = false)
         // we need to discard the wrapping "Twig_Node" for the "body" node
         $nested = $nested || ('Twig_Node' !== \get_class($node) && Node::class !== \get_class($node));
         foreach ($node as $k => $n) {
+            if ($n instanceof IfNode) {
+                continue;
+            }
+
             if (null !== $n && null === $this->filterBodyNodes($n, $nested)) {
                 $node->removeNode($k);
             }

tests/Fixtures/tags/if/empty_body.test

@@ -0,0 +1,22 @@
+--TEST--
+empty "if" body in child template
+--TEMPLATE--
+{% extends 'base.twig' %}
+
+{% set foo = '' %}
+
+{% if a is defined %}
+
+{% else %}
+    {% set foo = 'NOTHING' %}
+{% endif %}
+
+{% block content %}
+    {{ foo }}
+{% endblock %}
+--TEMPLATE(base.twig)--
+{% block content %}{% endblock %}
+--DATA--
+return []
+--EXPECT--
+  NOTHING