agent: ignore empty query

[droot]

With this PR, agent as well the terminal-ui will ignore empty user queries. Before this PR, we had a bug in the system where empty query will force the agent into an irrecoverable state where it will continue to make LLM request with empty query and keep on causing 400: invalid argument LLM API error.

And it is easy to get in this state especially when you are waiting for LLM to respond and end up pressing "ENTER" with no input.

An example:

>>>
E0813 21:27:30.319436   66139 conversation.go:501] "error reading streaming LLM response" err=<
        Error 400, Message: * GenerateContentRequest.contents[157].parts[0].data: required oneof field 'data' must have one initialized field
        , Status: INVALID_ARGUMENT, Details: []
 >
E0813 21:27:30.319541   66139 conversation.go:538] "error streaming LLM response" err=<
        Error 400, Message: * GenerateContentRequest.contents[157].parts[0].data: required oneof field 'data' must have one initialized field
        , Status: INVALID_ARGUMENT, Details: []
 >
Error: Error 400, Message: * GenerateContentRequest.contents[157].parts[0].data: required oneof field 'data' must have one initialized field

I will cut another release once this gets merged.

/cc @noahlwest @ShubyM

[noahlwest]

lgtm