The helper/schema diff process loses empty strings, causing them to show
up as unset (null) during apply. Besides failing to show as set by
GetOk, the absence of the value also triggers the schema to insert a
default value again during apply.
It would also be be preferable if the defaults weren't re-evaluated
again during ApplyResourceChange, but that would require a more invasive
patch to the field readers, and ensuring the empty string is stored in
the plan should block the default.
James Bardin