Fix issue where custom attributes with a null value cause an error when they are returned in the GraphQL API.
See: https://github.com/magento/magento2/issues/38884

@package magento/module-catalog-graph-ql

===================================================================
diff --git a/Model/Resolver/Product/ProductCustomAttributes.php b/Model/Resolver/Product/ProductCustomAttributes.php
--- a/Model/Resolver/Product/ProductCustomAttributes.php
+++ b/Model/Resolver/Product/ProductCustomAttributes.php	(date 1721216924752)
@@ -110,6 +110,9 @@
             if (is_array($attributeValue)) {
                 $attributeValue = implode(',', $attributeValue);
             }
+            if (!$attributeValue) {
+                continue;
+            }
             $customAttributes[] = [
                 'attribute_code' => $attributeCode,
                 'value' => $attributeValue