LS Client crash when passing `None` as the `api_key`

[GuilhermeFreire]

Describe the bug
If you pass None as your api_key and you don't have your LABEL_STUDIO_API_KEY env var set when creating a label_studio_sdk.Client, you'll get an AttributeError when trying to access credentials.api_key.

This happens because, the check for api_key being None and using credentials is wrong on client.py:

label-studio-sdk/label_studio_sdk/client.py

Lines 83 to 92 in 415049a

	if api_key is None and credentials is None:
	api_key = os.getenv('LABEL_STUDIO_API_KEY')
	if api_key is not None:
	credentials = ClientCredentials(api_key=api_key)
	self.api_key = (
	credentials.api_key
	if credentials.api_key
	else self.get_api_key(credentials)
	)

To Reproduce
Run the following without setting your LABEL_STUDIO_API_KEY env var.

>>> import label_studio_sdk
>>> label_studio_sdk.Client()

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/path/to/label_studio_sdk/client.py", line 90, in __init__
    if credentials.api_key
AttributeError: 'NoneType' object has no attribute 'api_key'

Expected behavior
Since you can't really do anything at this point (no api_key and no credentials), the SDK should just raise an error with a better error message. It's a small change and I could probably make a PR for it when I have some time 😅, but just thought of creating the issue in case anyone is able to fix this first.