Fix: Error of calculator when enter only one number with operator to get output

[SAGARSINGH-1]

Fix: #472

Checklist:

• [X ] I have read freeCodeCamp's contribution guidelines.
• [ X] My pull request has a descriptive title (not a vague title like Update index.md)

This pull request addresses an issue in the calculator application where invalid input expressions could lead to unexpected errors. It includes improvements to input validation and error handling to provide a better user experience.