fix: set proper cache key for singles when name is passed as None (f…

…rappe#18667)

[skip ci]

frappe/__init__.py

@@ -1115,7 +1115,7 @@ def can_cache_doc(args) -> str | None:
 		return
 
 	doctype = args[0]
-	name = doctype if len(args) == 1 else args[1]
+	name = doctype if len(args) == 1 or args[1] is None else args[1]
 
 	# Only cache if both doctype and name are strings
 	if isinstance(doctype, str) and isinstance(name, str):