fix: initialize streaming tool call arguments to empty string

When the first streaming chunk from an OpenAI-compatible provider
includes a tool call with function.name but no arguments field,
function.arguments is undefined. Subsequent chunks concatenate to
it via +=, producing "undefined{...}" which fails JSON parsing
and results in empty fn_args.

Initializing to "" on the first chunk prevents this.

Author: mattyhogan

src/providers/groq.ts

@@ -127,6 +127,9 @@ export class GroqProvider extends Provider {
 
                         if (!streamedToolCalls[index]) {
                             streamedToolCalls[index] = toolCall;
+                            if (streamedToolCalls[index].function && streamedToolCalls[index].function.arguments == null) {
+                                streamedToolCalls[index].function.arguments = '';
+                            }
                         } else {
                             if (streamedToolCalls[index].function) {
                                 streamedToolCalls[index].function.arguments +=

src/providers/openai.ts

@@ -118,6 +118,9 @@ export class OpenAIProvider extends Provider {
 
                         if (!streamedToolCalls[index]) {
                             streamedToolCalls[index] = toolCall;
+                            if (streamedToolCalls[index].function && streamedToolCalls[index].function.arguments == null) {
+                                streamedToolCalls[index].function.arguments = '';
+                            }
                         } else {
                             if (streamedToolCalls[index].function) {
                                 streamedToolCalls[index].function.arguments +=