[fix] validate preferred email if set in employee.py
diff --git a/erpnext/hr/doctype/employee/employee.py b/erpnext/hr/doctype/employee/employee.py
index 13aa524..a078ef2 100755
--- a/erpnext/hr/doctype/employee/employee.py
+++ b/erpnext/hr/doctype/employee/employee.py
@@ -157,7 +157,7 @@
 		delete_events(self.doctype, self.name)
 
 	def validate_prefered_email(self):
-		if not self.get(scrub(self.prefered_contact_email)):
+		if self.prefered_contact_email and not self.get(scrub(self.prefered_contact_email)):
 			frappe.msgprint(_("Please enter " + self.prefered_contact_email))