Fix white screen in article hero when image missing.

templates/article_pages/article--hero.html.twig

@@ -1,4 +1,7 @@
-{% set imagePath = file_url(content.field_article_highlight_image|field_target_entity.image.entity.uri.value) %}
+{% set imageValue = content.field_article_highlight_image|field_target_entity.image.entity.uri.value %}
+{% if imageValue %}
+  {% set imagePath = file_url(imageValue) %}
+{% endif %}
 {% set articleType = content.field_article_type|field_value %}
 {% set pubDate = content.field_publication_date_electroni|field_value %}
 
@@ -35,7 +38,7 @@
 
 
 
-<header style="background-image: linear-gradient(rgba(0, 0, 0, 0.5), rgba(0, 0, 0, 0.5)), url('{{ imagePath }}')" class="article-title-banner node__header paragraph-responsive-typography-enabled article-header">
+<header style="background-image: linear-gradient(rgba(0, 0, 0, 0.5), rgba(0, 0, 0, 0.5)){% if imagePath %}, url('{{ imagePath }}'){% endif %}" class="article-title-banner node__header paragraph-responsive-typography-enabled article-header">
   <div  class="container article-info--wrapper">
 
         {% block title_area %}