`locale` parameter cannot be `None`

[injust]

Overview Description

The type annotations for the locale parameter (e.g. in babel.numbers.format_currency() suggest that the value can be None, but this causes a TypeError in Locale.parse().

Steps to Reproduce

babel.numbers.format_currency(0, "USD", locale=None)

Actual Results

Traceback (most recent call last):
  File "<python-input-1>", line 1, in <module>
    babel.numbers.format_currency(0, "USD", locale=None)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^
  File ".venv/lib/python3.13/site-packages/babel/numbers.py", line 777, in format_currency
    locale = Locale.parse(locale)
  File ".venv/lib/python3.13/site-packages/babel/core.py", line 332, in parse
    raise TypeError(f"Unexpected value for identifier: {identifier!r}")
TypeError: Unexpected value for identifier: None

Expected Results

I would expect a value of None to use the default value, or None should be removed from the type annotations entirely.

Reproducibility

Additional Information

[bestpantusen]

If possible, I'd like to work on this please.

[tomasr8]

If possible, I'd like to work on this please.

Go for it! Feel free to ask if you need help :)