fix(client): check for empty ToolCalls before accessing

[stainless-app[bot]]

Fixes #437 - prevents panic when tool_calls is empty by adding length check before accessing delta.ToolCalls[0].Index

[coolbaluk]

@jacobzim-stl would this solve the issue here from my PR https://github.com/openai/openai-go/pull/469?

If so can we get it merged please, would like to start using v2 without syncing the fork : )