Hide default None value in cli_hide_none_type is set

[chovanecadam]

When a field defaults to None, displaying (default: null) provides no additional information to users. The absence of a default value already implies None/null for optional fields, and this matches the behavior of most CLI tools (argparse, click) where optional arguments don't display default: None. Users expect optional CLI arguments to default to None implicitly.

This change aligns pydantic-settings with standard CLI tool conventions (argparse, click) where None defaults are hidden. Also, this reduces visual noise in help messages:

class Settings(BaseSettings):
    model_config = SettingsConfigDict(
        cli_hide_none_type=True,
    )

    foo: Path | None
    bar: str | None
    baz: str | None = None
    timeout: int = 50
    config: Path | None = None

before

usage: main.py [-h] [--foo Path] [--bar str] [--baz str] [--timeout int] [--config Path]

options:
  -h, --help     show this help message and exit
  --foo Path     (required)
  --bar str      (required)
  --baz str      (default: null)
  --timeout int  (default: 50)
  --config Path  (default: null)

after

usage: main.py [-h] [--foo Path] [--bar str] [--baz str] [--timeout int] [--config Path]

options:
  -h, --help     show this help message and exit
  --foo Path     (required)
  --bar str      (required)
  --baz str
  --timeout int  (default: 50)
  --config Path

As this MR alters the behavior of cli_hide_none_type, you might prefer adding another flag to support this, e.g. cli_hide_none_type_defaults?

[hramezani]

Thanks @chovanecadam for this patch. We need a test for this.

@kschwab Could you please take a look when you have time?