[Minor Fix] Force toggle display Net Total field when default taxes are fetched (#15532)

* Minor fix for the problem that Net Total, Net Rate, Net Amount field would not be shown when default taxes were fetched

* Prevent net_rate from becoming NaN

* -Fixed problem that party_account_currency was not set in mapped document
-Fixed problem that discount amount was not recalculated (in mapped document)
-Fixed problem that set_dynamic_fields would work only on certain condition that led to showing Total (Company Currency) field even when using company currency

erpnext/controllers/taxes_and_totals.py

@@ -70,6 +70,7 @@ class calculate_taxes_and_totals(object):
 						if item.rate_with_margin > 0 else item.rate
 
 				item.net_rate = item.rate
+				item.discount_amount = item.price_list_rate - item.rate
 				item.amount = flt(item.rate * item.qty,	item.precision("amount"))
 				item.net_amount = item.amount