text: Title: Don't spit env vars if env_nested_delimiter=None
  Body: 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`:
  
  ```python
      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'
  ```