fix: set customer currency in pos_invoice if exists

if currency exists in the profile and customer currency doesn't exists still it will update currency to None, so update customer currency only if exists

(cherry picked from commit 041d52e)

erpnext/accounts/doctype/pos_invoice/pos_invoice.py

@@ -518,7 +518,7 @@ def set_pos_fields(self, for_validate=False):
 				selling_price_list = (
 					customer_price_list or customer_group_price_list or profile.get("selling_price_list")
 				)
-				if customer_currency != profile.get("currency"):
+				if customer_currency and customer_currency != profile.get("currency"):
 					self.set("currency", customer_currency)
 
 			else: