Reject explicitly empty index store type

[HUSTERGS]

Description

Explicitly setting index.store.type to an empty string currently bypasses the registered directory factory lookup. The create-index request is acknowledged, but shard creation later fails with no matching store type for [], leaving the index red.

This change distinguishes an omitted store type from an explicitly configured empty value:

• When index.store.type is absent, the internal empty default continues to select the default directory factory.
• When index.store.type is explicitly set to "", it enters the existing directory factory lookup and fails with IllegalArgumentException: Unknown store type [].
• Valid built-in and plugin-provided store types are unaffected.

The failure occurs while creating the temporary IndexService, before the index is added to the cluster state.

This change adds an internal-cluster test verifying that an explicitly empty store type is rejected during index creation.

Validation performed:

./gradlew :server:internalClusterTest \
  --tests "org.opensearch.action.admin.indices.create.CreateIndexIT.testExplicitEmptyStoreTypeIsRejected"

./gradlew :server:test \
  --tests "org.opensearch.index.IndexModuleTests"

./gradlew :server:spotlessJavaCheck

Related Issues

Resolves #22550

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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❌ Gradle check result for 0c7ecbe: null

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