list: send non-null "config" object to provider when not present in the list block

[dsa0x]

This PR ensures that when a list config isn't given a config block, terraform sends a non-null config to the provider RPC. Also, we want to validate the existence of the config block if any of its direct attribute or block is required.

Because we were already manually constructing a schema for the list block anyway, I updated that schema to make config required if its direct attribute are required.

Target Release

1.14.x

Rollback Plan

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

CHANGELOG entry

• This change is user-facing and I added a changelog entry.
• This change is not user-facing.

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