Always show composed_of field for composable index templates

Prior to e786cfa we inadvertently always added composable index
templates with `composed_of: []` beacuse
elastic@e786cfa#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 elastic#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

@@ -27,6 +27,7 @@
 import org.elasticsearch.xcontent.XContentParser;
 
 import java.io.IOException;
+import java.util.Collections;
 import java.util.List;
 import java.util.Map;
 import java.util.Objects;
@@ -273,7 +274,9 @@ public XContentBuilder toXContent(XContentBuilder builder, Params params, @Nulla
             builder.field(TEMPLATE.getPreferredName());
             this.template.toXContent(builder, params, rolloverConfiguration);
         }
-        if (this.componentTemplates != null) {
+        if (this.componentTemplates == null) {
+            builder.stringListField(COMPOSED_OF.getPreferredName(), Collections.emptyList());
+        } else {
             builder.stringListField(COMPOSED_OF.getPreferredName(), this.componentTemplates);
         }
         if (this.priority != null) {