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

This PR was submitted for the 6.0 branch but it was merged into the 5.4 branch instead.

Discussion
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[Form] Update create_form_type_extension.rst

Hello,
I think the condition `image_url is not null` is not enough as if we create a FileType form field without the `image_property` it causes an error. Adding `image_url is defined` is necessary  because the variable `image_url` is only defined and passed to the view when `image_property` is already defined.

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Commits
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85470c5 Update create_form_type_extension.rst

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 %}