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/test/java/org/elasticsearch/cluster/metadata/MetadataTests.java
@Override
        public Iterator<? extends ToXContent> toXContentChunked(ToXContent.Params params) {
            return Collections.emptyIterator();
        }
        @Override
        public Diff<Metadata.Custom> diff(Metadata.Custom previousState) {
            return null;
        }
        @Override
        public EnumSet<Metadata.XContentContext> context() {
            return null;
        }
        @Override
        public String getWriteableName() {
            return null;
        }
        @Override
        public TransportVersion getMinimalSupportedVersion() {
            return null;
        }
        @Override
        public void writeTo(StreamOutput out) throws IOException {
        }
    }
}