Return an empty variation if image does not exist

This prevents exceptions with `Impossible to access an attribute ("uri") on a null variable.` in Twig

(cherry picked from commit 3779361)

bundle/Templating/Twig/Extension/ImageRuntime.php

@@ -41,7 +41,7 @@ public function __construct(
      *
      * @return null|\eZ\Publish\SPI\Variation\Values\Variation
      */
-    public function getImageVariation(Field $field, string $variationName): ?Variation
+    public function getImageVariation(Field $field, string $variationName): Variation
     {
         /** @var \eZ\Publish\Core\FieldType\Image\Value $value */
         $value = $field->value;
@@ -60,6 +60,6 @@ public function getImageVariation(Field $field, string $variationName): ?Variati
             );
         }
 
-        return null;
+        return new Variation();
     }
 }