Add docs for operator : conditional - defaultIfEmpty

[ashwin-sureshkumar]

Add docs for operator defaultIfEmpty under group conditional

Below is minimum details needed

• [ ] name
• [ ] operatorType
• [ ] signature
• [ ] parameters
• [ ] short description
• [ ] Technical description
• [ ] Visual example (Marble diagram)
• [ ] Real world example (Non-framework)
• [ ] Related Operators

[hardikpthv]

working on it...