perf: don't use ifnull where it's not required (#36336)

ifnull isn't really required when doing `!= 'anything'` because if it's null then value will be falsy.
ifnull is only required when checking `= ''` if you treat `null = ''`

Actuall better fix would be make things explcitly non-nullable, then we won't ever have to add this on such fields.

ref: https://github.com/frappe/frappe/pull/21822

erpnext/manufacturing/doctype/bom_update_log/bom_updation_utils.py

@@ -161,7 +161,7 @@ def get_leaf_boms() -> List[str]:
 		"""select name from `tabBOM` bom
 		where docstatus=1 and is_active=1
 			and not exists(select bom_no from `tabBOM Item`
-				where parent=bom.name and ifnull(bom_no, '')!='')"""
+				where parent=bom.name and bom_no !='')"""
 	)