minor #16599 [Form] Update create_form_type_extension.rst (javiereguiluz)

This PR was merged into the 4.4 branch.

Discussion
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[Form] Update create_form_type_extension.rst

backport #16506 to `4.4`

Commits
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ac8a988 minor #16506 [Form] Update create_form_type_extension.rst (issamkhadiri1989)

Author: javiereguiluz

form/create_form_type_extension.rst

@@ -192,7 +192,7 @@ Specifically, you need to override the ``file_widget`` block:
 
     {% block file_widget %}
         {{ block('form_widget') }}
-        {% if image_url is not null %}
+        {% if image_url is defined and image_url is not null %}
             <img src="{{ asset(image_url) }}"/>
         {% endif %}
     {% endblock %}