[bug] Dynaconf does not support null values in yaml

[Bahus]

Describe the bug
If you specify a null value for a variable in a yaml config file and when you try to use this variable you will get "variable does not exists" error.

The same issue should appear when using nulls in vault (since it's a JSON).

To Reproduce
The following python snippet can be used to reproduce the issue:

from dynaconf.base import Settings
settings = Settings()
settings['name'] = 'Hello'
settings['nulled_name'] = None

print(settings['name'])
print(settings['nulled_name'])

The output is:

Hello
Traceback (most recent call last):
  File ".../dynaconf/base.py", line 222, in __getitem__
    raise KeyError("{0} does not exists".format(item))
KeyError: 'nulled_name does not exists'

[lobocode]

I believe this has something to do with the version of YAML used in dynaconf. The Null value in Yaml can be expressed with null,~. If you use settings ['nulled_name'] = None, you are using Python values ​​to set null to YAML. That is, the syntax is incorrect. However, even using the correct syntax, it will not work. In this case, try to use settings['nulled_name'] = " " https://yaml.org/type/null.html .

[Bahus]

@lobocode hi, the "to reproduce" section script shows the problem in the dynaconf core, which in turns affects how dynaconf parses YAML configs (you can reproduce it with value: null or value: ~ in an arbitrary YAML file). I don't really wan't to replace nullable values with spaces, since it may lead to unintended consequences in my application.