Correctly return tool input when creating the message

For tool calls which had no arguments, the tool input was returned as empty string which
does not work for some models.

fixes #15455

Author: eneufeld

packages/ai-chat/src/common/chat-model.ts

@@ -1043,7 +1043,7 @@ export class ToolCallChatResponseContentImpl implements ToolCallChatResponseCont
             actor: 'ai',
             type: 'tool_use',
             id: this.id ?? '',
-            input: (this.arguments && JSON.parse(this.arguments)) ?? undefined,
+            input: this.arguments && this.arguments.length !== 0 ? JSON.parse(this.arguments) : {},
             name: this.name ?? ''
         }, {
             actor: 'user',