fix: set default value in set_value as None to avoid error.
fix: set default value in set_value as None to avoid error.

```
frappe.db.set_value('Call Log', log.name, {
			fieldname: doc.name,
			display_name_field: doc.get_title()
		}, update_modified=False)
```
this code should not fail if value is not pass because value is passed in the dict.
mhbu50/frappe,mhbu50/frappe,vjFaLk/frappe,StrellaGroup/frappe,saurabh6790/frappe,frappe/frappe,yashodhank/frappe,adityahase/frappe,vjFaLk/frappe,frappe/frappe,frappe/frappe,adityahase/frappe,StrellaGroup/frappe,saurabh6790/frappe,almeidapaulopt/frappe,mhbu50/frappe,yashodhank/frappe,vjFaLk/frappe,almeidapaulopt/frappe,mhbu50/frappe,almeidapaulopt/frappe,saurabh6790/frappe,adityahase/frappe,almeidapaulopt/frappe,yashodhank/frappe,StrellaGroup/frappe,vjFaLk/frappe,adityahase/frappe,yashodhank/frappe,saurabh6790/frappe