Prevent an endless diff loop for deployments with no trust defined

[tobio]

Description

Does what it says on the tin. I can't create a deployment in this state, however I've tested the code against the response body of the deployment mentioned in the linked issue.

Null state values are ignored by UseStateForUnknown, which leads to endless diff loops when trust settings are nil.

Related Issues

Fixes #686

Motivation and Context

How Has This Been Tested?

Unit tests, manually

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (improves code quality but has no user-facing effect)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation

Readiness Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

[tobio]

Acceptance test failure appears unrelated.

[wandergeek]

~/work/terraform-provider-ec pr/687                                                                                                                                                                     ✘ INT  base
❯ make check-license
-> Running license check...
ec/ecresource/deploymentresource/elasticsearch/v2/elasticsearch_trust_account_test.go: is missing the license header
ec/ecresource/deploymentresource/elasticsearch/v2/elasticsearch_trust_external_test.go: is missing the license header
make: *** [build/Makefile.lint:19: check-license] Error 1

Missing license header. LGTM otherwise.