Always show `composed_of` field for composable index templates (#105315) (#105572)

* Always show `composed_of` field for composable index templates

Prior to e786cfa7061b427cf6185ad907069838dd679574 we inadvertently always added composable index
templates with `composed_of: []` beacuse
https://github.com/elastic/elasticsearch/commit/e786cfa7061b427cf6185ad907069838dd679574#diff-5081302eb39033199deb1977d544d1cd7867212a92b8d77e0aa0ded361272b11L618-L630
created a new `ComposableIndexTemplate` from an existing one, and the `.composedOf()` field returned
an empty list of no component templates were provided:

https://github.com/elastic/elasticsearch/blob/89e714ee5dc60db8b4979ab6372ff767e108e9da/server/src/main/java/org/elasticsearch/cluster/metadata/ComposableIndexTemplate.java#L172-L177

This meant that before 8.12.0 we would always show `composed_of: []` for composable index templates.
This commit recreates this behavior, and always displays the empty list even if no component
templates are used by a composable index template.

Resolves #104627

docs/changelog/105315.yaml

@@ -0,0 +1,6 @@
+pr: 105315
+summary: Always show `composed_of` field for composable index templates
+area: Indices APIs
+type: bug
+issues:
+ - 104627

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/indices.get_index_template/10_basic.yml

@@ -188,3 +188,16 @@ setup:
                   type: keyword
             lifecycle:
               data_retention: "30d"
+
+---
+"Get index template always shows composed_of":
+  - skip:
+      version: " - 8.12.99"
+      reason: "A bug was fixed in 8.13.0 to make `composed_of` always returned"
+
+  - do:
+      indices.get_index_template:
+        name: test
+
+  - match: {index_templates.0.name: test}
+  - match: {index_templates.0.index_template.composed_of: []}

server/src/main/java/org/elasticsearch/cluster/metadata/ComposableIndexTemplate.java

@@ -125,7 +125,7 @@ public class ComposableIndexTemplate implements SimpleDiffable<ComposableIndexTe
     ) {
         this.indexPatterns = indexPatterns;
         this.template = template;
-        this.componentTemplates = componentTemplates;
+        this.componentTemplates = componentTemplates == null ? List.of() : componentTemplates;
         this.priority = priority;
         this.version = version;
         this.metadata = metadata;