(V1) Fix | Use of null in MockResponse data

[Sammyjo20]

Fixes #213

This PR fixes an issue with v1's mock responses if you were to use null as the data of the mock response, it would throw an exception.

[Sammyjo20]

Self review

[what-the-diff]

PR Summary 😊

• Improved getFormattedData method
  The getFormattedData method now returns null if there's no data, making it smarter! 🧠
• Changed getHeader return type
  The getHeader return type is updated from array to string, and it returns an empty string if no header is found. This makes it more consistent. 🌟
• Fixed comment typo in CollectsData
  A small typo fix in the docblock comment to improve code readability. 📚
• Added a test case for MockClient constructor
  A new test case is added to check if null as the response body is treated like an empty string. This helps ensure code reliability. 🧪