Change tool_calls to be an Option in response (#13778)

Here is an image of my now getting assistance responses!

![2024-07-03_08-45-37_swappy](https://github.com/zed-industries/zed/assets/20910163/904adc51-cb40-4622-878e-f679e0212426)

I ended up adding a function to handle the use case of not serializing
the tool_calls response if it is either null or empty to keep the
functionality of the existing implementation (not deserializing if vec
is empty). I'm sorta a noob, so happy to make changes if this isn't done
correctly, although it does work and it does pass tests!

Thanks a bunch to [amtoaer](https://github.com/amtoaer) for pointing me
in the direction on how to fix it.

Release Notes:

- Fixed some responses being dropped from OpenAI-compatible providers
([#13741](https://github.com/zed-industries/zed/issues/13741)).

crates/collab/src/rpc.rs

@@ -4462,6 +4462,7 @@ async fn complete_with_open_ai(
                         tool_calls: choice
                             .delta
                             .tool_calls
+                            .unwrap_or_default()
                             .into_iter()
                             .map(|delta| proto::ToolCallDelta {
                                 index: delta.index as u32,