ToolCall::arguments() throws TypeError when model returns JSON null string

[obalaweb]

Bug Report

Summary

ToolCall::arguments() declares : array as its return type but can return
null when the model emits the literal JSON string "null" in the
function.arguments field, causing a TypeError at the call site.

Steps to Reproduce

1. Register any tool with no required parameters
2. Use a Groq provider with a Llama model (e.g. llama-3.3-70b-versatile)
3. Ask the model something that triggers the no-arg tool call
4. Prism attempts to execute the tool and throws

Error

  TypeError: Prism\Prism\ValueObjects\ToolCall::arguments(): Return value
  must be of type array, null returned

  #0 vendor/prism-php/prism/src/Concerns/CallsTools.php:137
  #1 vendor/prism-php/prism/src/ValueObjects/ToolCall.php:42

Root Cause

Some models emit "arguments": "null" (valid JSON representing a null value)
instead of "arguments": "{}" for no-arg tool calls.

json_decode('null', true) is valid and returns PHP null, which violates
the : array return type.

Expected Behaviour

arguments() always returns an array. JSON null should be treated as
"no arguments" and return [].

Suggested Fix

$decoded = json_decode($arguments, true, flags: JSON_THROW_ON_ERROR);
return is_array($decoded) ? $decoded : [];

Environment

- Provider: Groq
- Model: llama-3.3-70b-versatile
- Prism: v0.99.22
                                                                                                                      
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