fix: we want DF.Literal[None], not DF.LiteralNone (#25394)

Signed-off-by: Akhil Narang <me@akhilnarang.dev>
(cherry picked from commit 1896117)

frappe/types/exporter.py

@@ -178,7 +178,7 @@ def _generic_parameters(self, field) -> str | None:
 		elif field.fieldtype == "Select":
 			if not field.options:
 				# Could be dynamic
-				return "None"
+				return "[None]"
 			options = [o.strip() for o in field.options.split("\n")]
 			return json.dumps(options)