Clearing the Rewrite port field makes AntD InputNumber write null into the
form store. The tunnel schema declared rewritePort as PortSchema.optional(),
which accepts undefined but not null, so saving (or the JSON tab reflecting
null) failed validation with "settings.rewritePort — Invalid input".
Accept null and collapse it to undefined so the field is simply omitted from
the serialized payload, matching the behavior of deleting the key by hand.
The trailing .optional() keeps the key optional in the inferred type.
Closes#5516
Rouzbeh†