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().

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.
Merge pull request #97 from UFAL-DSG/fix_silence_in_tts

Preventing "silence()" from being passed to TTS