control no files per billing_run_id

app/models.py

@@ -79,6 +79,8 @@ def get_invoice_oc(con, billing_run_uuid):
     columns = ['description_cat' ,'code', 'preu', 'amount_tax', 'amount_with_tax', 'tax_percent', 'group_code',
                'tax_code', 'billing_account_code']
     registries = con.execute(statement)
+    if registries.rowcount == 0:
+        return 'NO DATA'
 
     df = DataFrame(registries.fetchall())
     df.columns = registries.keys()