When calling createEmptyBlocks, we only intend to process legacy SDK
blocks (NestingList or NestingSet), but the function also needs to pass
through NestingSingle block types untouched. To do so we must only look
up the container's element type after we've established that the block
is backed by a container.
In order to determine if we need to re-read a data source during plan,
we need to compare the newly evaluated configuration with the stored
state. To do that we create a ProposedNewVal, which if there are no
changes, should match the existing state exactly.
A problem arises if the remote data source contains any blocks, and they
are not set in the configuration. Terraform always decodes configuration
blocks as empty containers, however the legacy SDK cannot correctly
handle empty blocks and may return a null block which is saved to the
state. In order to correctly make the comparison for planning, we need
to reify those null blocks as empty containers in the cty value.
The createEmptyBlocks helper converts any null NestingList or NestingSet
blocks to empty list or set cty values. We only need to be concerned
with List and Set, because those are the only types that can be defined
with the legacy SDK. In hindsight these could have been normalized in
the legacy SDK shims had this problem been uncovered earlier, but for the
sake of compatibility we will now normalize these in core.
