[8.x] Fix Client\Response throw method

[rodrigopedra]

Closes #34591

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Commit d034e2c changed the Illuminate\Http\Client\Response@throw method to accept an optional callback. But code started failing when the throw method was being called without any parameters.

According to PHP docs on func_get_arg https://www.php.net/manual/en/function.func-get-arg.php

Generates a warning if called from outside of a user-defined function, or if arg_num is greater than the number of arguments actually passed.

This PR fixes that by changing that statement to $callback = func_get_args()[0] ?? null.

While testing locally I noticed the same commit also created an $exception variable inside a if block and throw it outside. In the case a $callback was not provided the code would fail due to a undefined $exception variable. This PR addresses that case too.