fix:Add shipping charges to taxes only if applicable

(cherry picked from commit e8bc4f12a734ff286a008e6b31dffa9c6c946365)
diff --git a/erpnext/accounts/doctype/shipping_rule/shipping_rule.py b/erpnext/accounts/doctype/shipping_rule/shipping_rule.py
index 7e51299..792e7d2 100644
--- a/erpnext/accounts/doctype/shipping_rule/shipping_rule.py
+++ b/erpnext/accounts/doctype/shipping_rule/shipping_rule.py
@@ -71,7 +71,8 @@
 		if doc.currency != doc.company_currency:
 			shipping_amount = flt(shipping_amount / doc.conversion_rate, 2)
 
-		self.add_shipping_rule_to_tax_table(doc, shipping_amount)
+		if shipping_amount:
+			self.add_shipping_rule_to_tax_table(doc, shipping_amount)
 
 	def get_shipping_amount_from_rules(self, value):
 		for condition in self.get("conditions"):