Preventing "silence()" from being passed to TTS

In some cases, the DM can output DAs like `notunderstood()&silence()` since `silence()` is sometimes returned by [`get_limited_context_help()`](https://github.com/UFAL-DSG/alex/blob/master/alex/applications/PublicTransportInfoCS/hdc_policy.py#L1142).

These are then not caught by NLG (since it's not only `silence()`) and `silence()` makes its way to TTS.

Introducing an empty template for `silence()` will effectively strip these DAIs from the output.