fix for 'NoneType' object has no attribute 'tool_calls' issue

[arty-hlr]

See https://github.com/TheR1D/shell_gpt/issues/662

Proposed fix by checking if delta is not None and if so just continuing the for loop. Another option would be to add a try catch around the delta.get("tool_calls") if use_litellm else delta.tool_calls. Let me know!