A composable index template with no `template` defined in the body is mistakingly always assumed to not be a time series template. Even if it refers to a component template that has the `index.mode` setting set to `time_series` and the component template defines mappings with dimension fields or routing paths.

Incorrectly this causes to create a data stream with a tsdb index *without* time series start and end time settings set. The fact that this can happen is another bug and is being tracked #98833.

What should happen in this case is that a tsdb data stream is created with a valid tsdb backing index (with correctly generated time series start and end time settings).

This bug was found as part investigating #98792 and is the cause of that bug.
server/src/main/java/org/elasticsearch/cluster/metadata/Metadata.java
templates = DiffableUtils.readImmutableOpenMapDiff(in, DiffableUtils.getStringKeySerializer(), TEMPLATES_DIFF_VALUE_READER);
            customs = DiffableUtils.readImmutableOpenMapDiff(in, DiffableUtils.getStringKeySerializer(), CUSTOM_VALUE_SERIALIZER);
            if (in.getTransportVersion().onOrAfter(TransportVersion.V_8_4_0)) {
                reservedStateMetadata = DiffableUtils.readJdkMapDiff(
                    in,
                    DiffableUtils.getStringKeySerializer(),
                    RESERVED_DIFF_VALUE_READER
                );
            } else {
                reservedStateMetadata = DiffableUtils.emptyDiff();
            }
        }
        @Override
        public void writeTo(StreamOutput out) throws IOException {
            if (out.getTransportVersion().onOrAfter(NOOP_METADATA_DIFF_SAFE_VERSION)) {
                out.writeBoolean(empty);
                if (empty) {
                    
                    return;
                }
            } else if (out.getTransportVersion().onOrAfter(NOOP_METADATA_DIFF_VERSION)) {
                
                out.writeBoolean(false);
            }
            out.writeString(clusterUUID);
            out.writeBoolean(clusterUUIDCommitted);
            out.writeLong(version);
            coordinationMetadata.writeTo(out);
            transientSettings.writeTo(out);
            persistentSettings.writeTo(out);