Fix NPE in deprecation API (#121263)

**Reproduction path**

1. Create an composable index template that does not have a `template` section.
2. Set some settings in `deprecation.skip_deprecated_settings`
3. Run the deprecation API `GET _migration/deprecations?error_trace`
4. **Result:** we receive an error `500` with the following error and stack trace:

```
....
"reason": "Cannot invoke \"org.elasticsearch.cluster.metadata.Template.settings()\" because \"template\" is null",
"stack_trace": "java.lang.NullPointerException: Cannot invoke \"org.elasticsearch.cluster.metadata.Template.settings()\" because \"template\" is null
org.elasticsearch.xpack.deprecation.DeprecationInfoAction.lambda$removeSkippedSettings$9(DeprecationInfoAction.java:408)
....
```

**Fix** There was a typo when we were performing the null-check, we used
`templateName` instead of `template`. In this PR we fix this and we
extend the current test to capture this case as well. 

The bug is not released so it's marked as a non-issue.

Author: gmarouli

x-pack/plugin/deprecation/src/main/java/org/elasticsearch/xpack/deprecation/DeprecationInfoAction.java

@@ -353,7 +353,7 @@ public static DeprecationInfoAction.Response from(
     }
 
     /**
-     *
+     * Removes the skipped settings from the selected indices and the component and index templates.
      * @param state The cluster state to modify
      * @param indexNames The names of the indexes whose settings need to be filtered
      * @param skipTheseDeprecatedSettings The settings that will be removed from cluster metadata and the index metadata of all the
@@ -405,7 +405,7 @@ private static ClusterState removeSkippedSettings(ClusterState state, String[] i
             String templateName = entry.getKey();
             ComposableIndexTemplate indexTemplate = entry.getValue();
             Template template = indexTemplate.template();
-            if (templateName == null || template.settings() == null || template.settings().isEmpty()) {
+            if (template == null || template.settings() == null || template.settings().isEmpty()) {
                 return Tuple.tuple(templateName, indexTemplate);
             }
             return Tuple.tuple(

x-pack/plugin/deprecation/src/test/java/org/elasticsearch/xpack/deprecation/DeprecationInfoActionResponseTests.java

@@ -362,7 +362,14 @@ public void testRemoveSkippedSettings() {
             .put(IndexMetadata.builder("test").settings(inputSettings).numberOfShards(1).numberOfReplicas(0))
             .put(dataStreamIndexMetadata, true)
             .put(DataStream.builder("ds-test", List.of(dataStreamIndexMetadata.getIndex())).build())
-            .indexTemplates(Map.of("my-index-template", indexTemplate))
+            .indexTemplates(
+                Map.of(
+                    "my-index-template",
+                    indexTemplate,
+                    "empty-template",
+                    ComposableIndexTemplate.builder().indexPatterns(List.of("random")).build()
+                )
+            )
             .componentTemplates(Map.of("my-component-template", componentTemplate))
             .persistentSettings(inputSettings)
             .build();
@@ -391,7 +398,11 @@ public void testRemoveSkippedSettings() {
                 .componentTemplates()
                 .values()
                 .forEach(template -> visibleComponentTemplateSettings.set(template.template().settings()));
-            cs.metadata().templatesV2().values().forEach(template -> visibleIndexTemplateSettings.set(template.template().settings()));
+            cs.metadata().templatesV2().values().forEach(template -> {
+                if (template.template() != null && template.template().settings() != null) {
+                    visibleIndexTemplateSettings.set(template.template().settings());
+                }
+            });
             return Map.of();
         }));