text-followup remove response_full tag if LLM has nothing to say

[jeffbl]

Right now, the LLM prompt for text-followup instructs that the LLM should populate the response_full tag with something like "I do not have any further details to provide". After conversation with @JRegimbal , this is not great behavior since then a future application would need to parse this (and it may vary slightly if the LLM is not paying proper attention), to know that there is nothing meaningful in this tag. The schema now defines the response_full tag as optional, and this work item is to change the LLM prompt to simply omit this tag entirely if the LLM itself does not think it can produce anything meaningful.