[update] fix null header value

wizard/import_provider_product_wizard.py

@@ -50,7 +50,10 @@ class ImportProviderProductWizard(models.TransientModel):
         ]
         product_fields_dict = dict(product_fields)
         # Header du fichier
-        xlsx_header = {cell.value: cell.column for cell in ws[1]}
+        xlsx_header = {}
+        for cell in ws[1]:
+            if cell.value:
+                xlsx_header[cell.value] = cell.column
 
         # Gestion du nom des colonnes
         column_field = []