do not merge - demo that empty strings are used in context of Azure b…

[SarahFrench]

Do not merge - illustrative purposes only.

Target Release

1.15.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.

[SarahFrench]

Test run output:

% go test -v github.com/hashicorp/terraform/internal/backend/remote-state/azure -run=TestBackendConfig_emptyStrings
=== RUN   TestBackendConfig_emptyStrings
    backend_test.go:63: TestBackendConfig on *azure.Backend with configtesting.synthBody{Filename:"<TestWrapConfig>", Values:map[string]cty.Value{"access_key":cty.StringVal("QUNDRVNTX0tFWQ0K"), "container_name":cty.StringVal("tfcontainer"), "key":cty.StringVal("state"), "snapshot":cty.False, "storage_account_name":cty.StringVal("tfaccount")}}
    backend_test.go:83: TestBackendConfig on *azure.Backend with configtesting.synthBody{Filename:"<TestWrapConfig>", Values:map[string]cty.Value{"access_key":cty.StringVal("QUNDRVNTX0tFWQ0K"), "container_name":cty.StringVal("tfcontainer"), "key":cty.StringVal("state"), "snapshot":cty.False, "storage_account_name":cty.StringVal("tfaccount"), "subscription_id":cty.StringVal("")}}
--- PASS: TestBackendConfig_emptyStrings (0.00s)
PASS
ok      github.com/hashicorp/terraform/internal/backend/remote-state/azure      0.361s