Don't spit env vars if env_nested_delimiter=None

[gsakkis]

By default env_nested_delimiter=None but this value is not special-cased, leading to surprising (if not ourtight buggy) behavior with case_sensitive=True:

    class Subsettings(BaseSettings):
        foo: str

    class Settings(BaseSettings):
        subsettings: Subsettings

        model_config = SettingsConfigDict(case_sensitive=True)

    env.set('subsettingsNonefoo', 'abc')
    s = Settings()
    assert s.subsettings.foo == 'abc'

[hramezani]

Thanks @gsakkis for reporting this issue.

The above example code that you provided returns pydantic_settings.sources.SettingsError before your last commit. that behavior was wrong but could be fixed by putting the below code at the beginning of function:

        if not self.env_nested_delimiter:
            return {}

This block of code can also fix the problem after you commit but I am afraid your change breaks people's code.
It would be great if you can take a look and provide a fix otherwise I will make a fix later.