Allow empty trigger sentence responses in conversations

[mib1185]

Proposed change

When setting set_conversation_response to an empty string (to avoid any spoken response), the sentence trigger was processed as expected, but the empty response text was considered as False so it still forwarded the request to llm

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[home-assistant]

Hey there @balloob, @synesthesiam, mind taking a look at this pull request as it has been labeled with an integration (assist_pipeline) you are listed as a code owner for? Thanks!

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