How to concatenate strings in GROQ and handle null values using coalesce function.

{
 firstName: "A",
 lastName: "B",
 fullName: firstName + lastName
}

// output: fullName: A B 

{
 firstName: NULL,
 lastName: "B",
 fullName: firstName + lastName
}

//output: NULL
Expected: output: B

coalesce(firstname, lastname)

'fullName': coalesce(firstName, "") + coalesce(lastName,"")