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` anyway). Oops.
Test Plan:
* Saved an empty text field and saw the delete work properly and NOT get
re-added.
* Put `null` in the text field, and saved successfully.
Reviewers: epriestley
Reviewed By: epriestley
CC: aran, Korvin
Differential Revision: https://secure.phabricator.com/D4300
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