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

**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:
https://github.com/HumanSignal/label-studio-sdk/blob/415049a6834db4f8cff4dec3e72755c26f6a2c38/label_studio_sdk/client.py#L83-L92

**To Reproduce**
Run the following without setting your `LABEL_STUDIO_API_KEY` env var.
```python
>>> import label_studio_sdk
>>> label_studio_sdk.Client()
```
```
Traceback (most recent call last):
File "", line 1, in
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.