perf: Skip updating defaults when nothing has changed (#29036)

ERPNext updates defaults everytime settings are changed but there's no
need to do it when value itself hasn't changed.

IDK if there are things that depend on this weird behaviour, there
shouldn't be any **ideally**.

Feel free to revert if it breaks.

frappe/defaults.py

@@ -4,6 +4,7 @@
 import frappe
 from frappe.cache_manager import clear_defaults_cache, common_default_keys
 from frappe.query_builder import DocType
+from frappe.utils.data import cstr
 
 # Note: DefaultValue records are identified by parent (e.g. __default, __global)
 
@@ -154,14 +155,17 @@ def set_default(key, value, parent, parenttype="__default"):
 	:param parent: Usually, **User** to whom the default belongs.
 	:param parenttype: [optional] default is `__default`."""
 	table = DocType("DefaultValue")
-	key_exists = (
+	current_value = (
 		frappe.qb.from_(table)
 		.where((table.defkey == key) & (table.parent == parent))
-		.select(table.defkey)
+		.select(table.defvalue)
 		.for_update()
-		.run()
+		.run(as_dict=True)
 	)
-	if key_exists:
+	if current_value:
+		if current_value[0].defvalue == cstr(value):
+			# Nothing has changed
+			return
 		frappe.db.delete("DefaultValue", {"defkey": key, "parent": parent})
 	if value is not None:
 		add_default(key, value, parent)