appstream_fleet: Fix empty domain_join bug

[YakDriver]

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Closes #24798

Output from acceptance testing:

% make testacc TESTS=TestAccAppStreamFleet PKG=appstream
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./internal/service/appstream/... -v -count 1 -parallel 20 -run='TestAccAppStreamFleet'  -timeout 180m
=== RUN   TestAccAppStreamFleetStackAssociation_basic
=== PAUSE TestAccAppStreamFleetStackAssociation_basic
=== RUN   TestAccAppStreamFleetStackAssociation_disappears
=== PAUSE TestAccAppStreamFleetStackAssociation_disappears
=== RUN   TestAccAppStreamFleet_basic
=== PAUSE TestAccAppStreamFleet_basic
=== RUN   TestAccAppStreamFleet_disappears
=== PAUSE TestAccAppStreamFleet_disappears
=== RUN   TestAccAppStreamFleet_completeWithStop
=== PAUSE TestAccAppStreamFleet_completeWithStop
=== RUN   TestAccAppStreamFleet_completeWithoutStop
=== PAUSE TestAccAppStreamFleet_completeWithoutStop
=== RUN   TestAccAppStreamFleet_withTags
=== PAUSE TestAccAppStreamFleet_withTags
=== RUN   TestAccAppStreamFleet_emptyDomainJoin
=== PAUSE TestAccAppStreamFleet_emptyDomainJoin
=== CONT  TestAccAppStreamFleetStackAssociation_basic
=== CONT  TestAccAppStreamFleet_completeWithStop
=== CONT  TestAccAppStreamFleet_withTags
=== CONT  TestAccAppStreamFleet_completeWithoutStop
=== CONT  TestAccAppStreamFleet_basic
=== CONT  TestAccAppStreamFleet_emptyDomainJoin
=== CONT  TestAccAppStreamFleet_disappears
=== CONT  TestAccAppStreamFleetStackAssociation_disappears
--- PASS: TestAccAppStreamFleetStackAssociation_disappears (889.77s)
--- PASS: TestAccAppStreamFleet_disappears (891.87s)
--- PASS: TestAccAppStreamFleetStackAssociation_basic (936.04s)
--- PASS: TestAccAppStreamFleet_emptyDomainJoin (944.70s)
--- PASS: TestAccAppStreamFleet_basic (978.95s)
--- PASS: TestAccAppStreamFleet_completeWithoutStop (987.97s)
--- PASS: TestAccAppStreamFleet_withTags (1095.04s)
--- PASS: TestAccAppStreamFleet_completeWithStop (1774.21s)
PASS
ok  	github.com/hashicorp/terraform-provider-aws/internal/service/appstream	1775.730s

[github-actions]

This functionality has been released in v4.28.0 of the Terraform AWS Provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!

[github-actions]

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