[FIX] account_avatax_oca: only override exemption if provided in Odoo

If no exemption info is provided in Odoo, the API call sends None.
This causes and override on exemption information that might exist in
the Avatax side.

account_avatax_oca/models/avatax_rest_api.py

@@ -287,14 +287,20 @@ def get_tax(
             "referenceCode": reference_code,
             "salespersonCode": salesman_code and salesman_code[:25] or None,
             "reportingLocationCode": location_code,
-            "entityUseCode": customer_usage_type,
-            "exemptionNo": exemption_no,
             "description": doc_code or "Draft",
             "date": doc_date,
             "code": doc_code,
             "type": doc_type,
             "commit": commit,
         }
+        # Only override exemption if provided from Odoo
+        if customer_usage_type or exemption_no:
+            create_transaction.update(
+                {
+                    "entityUseCode": customer_usage_type,
+                    "exemptionNo": exemption_no,
+                }
+            )
         if is_override and invoice_date:
             create_transaction.update(
                 {