R1:908253f1db01

Fix two bugs with Config's Edit controller.

Summary:
* When we restored to the default value, we did, in fact delete the row from the
database, but then a few lines later down, we saved it again. This patch causes
the controller to return early on delete, like it was supposed to do to begin
with.
* When checking the user's input value for `null` (since PHP's JSON encoder will
return `null` on failure), check the value that the user gave, not the value
that we default to (which is often `null`…