prevent an empty string from being lost

[jbardin]

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.

Fixes #21760

[apparentlymart]

I feel like I've lost enough context in the intervening weeks to not be able to fully reason about what this is gonna do, but I trust that you have more context loaded and have thought this through, and so my review here focused on "does this code look like it does what it was intending to do". 😀

[jbardin]

Forgot the unit test for that change.
The behavioral change is quite small, a plan cannot replace an empty string with null, where this used to only be enforced for apply. This also further limits the check to empty strings, since that is the ambiguous flatmap case.

[ghost]

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