Fix tool call coalescing

The previous code seemed to assume that the tool call index property
started at 0, but Anthropic sometimes returns them starting at 1.
This was causing an extra null-ish tool call to be materialized.

litellm/main.py

@@ -5094,7 +5094,7 @@ def stream_chunk_builder(
         name = None
         type = None
         tool_calls_list = []
-        prev_index = 0
+        prev_index = None
         prev_id = None
         curr_id = None
         curr_index = 0
@@ -5120,6 +5120,8 @@ def stream_chunk_builder(
                         name = tool_calls[0].function.name
                     if tool_calls[0].type:
                         type = tool_calls[0].type
+            if prev_index is None:
+                prev_index = curr_index
             if curr_index != prev_index:  # new tool call
                 combined_arguments = "".join(argument_list)
                 tool_calls_list.append(
@@ -5138,6 +5140,7 @@ def stream_chunk_builder(
         tool_calls_list.append(
             {
                 "id": id,
+                "index": curr_index,
                 "function": {"arguments": combined_arguments, "name": name},
                 "type": type,
             }