refactor: Refactor validation in SignUp.jsx

[nikohoffren]

Changes:

• Replaced if-else conditions with a for loop to check for empty fields.
• Mapped error messages to their respective fields in an array to reduce code repetition.
• Improved the readability of the validation logic in the signup function.

The new implementation is cleaner and easier to read and the changes reduce the amount of repeated code.
Also future modifications to the fields or the error messages can be easily handled by updating the array, improving the maintainability of the code. This solution should not affect the functionality of the code.