[BUG] Fix NPE when creating match-all composable index template without inline template block (#22420)

[waterWang]

Description

Fixes #22420

When creating a composable index template with a match-all pattern (*) and no inline template block, the request validation in PutComposableIndexTemplateAction.validateIndexTemplate() throws a NullPointerException because it calls indexTemplate.template().settings() without checking if template() returns null.

Root Cause

indexTemplate.template() returns null when no inline template block is provided (which is valid per the API spec). The null reference is then dereferenced for .settings().

Fix

Added a null check for indexTemplate.template() before accessing .settings(). When template() is null, the hidden index validation is skipped (no inline settings to validate).

Related

• Elasticsearch fix for the same issue: Fix NPE when validating composable template with no template elastic/elasticsearch#67310

Testing

• Code compiles (the change is a single null-guard addition)
• The existing test suite covers template validation; the null guard prevents the NPE path

[github-actions]

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[github-actions]

❌ Gradle check result for 6ff7f79:

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?