Building URLs with query parameters is something you'll do constantly when working with APIs. Let's build a utility function that handles all the edge cases.

Your Task:

• Create a function buildQueryString(params) that takes an object
• Return a properly formatted query string starting with ?
• Use Object.entries() to iterate over the object
• Use encodeURIComponent() for proper URL encoding
• Filter out null or undefined values
• Example: { name: "Alice", age: 25 } → "?name=Alice&age=25"