[API-7676]: Validate apiKey is not null when creating a Client

[esingin-sift]

Purpose

https://sift.atlassian.net/browse/API-7676

Summary

• Validate apiKey is not null when creating a Client

Testing

Checklist

• The change was thoroughly tested manually
• The change was covered with unit tests
• The change was tested with real API calls (if applicable)
• Necessary changes were made in the integration tests (if applicable)
• New functionality is reflected in README

[sbogolii-sift]

LGTM