[Improvement]: Change bal add generated code to adhere to best practices

[gayaldassanayake]

Description

The current generated code when we add a module using bal add looks like,

public function hello(string name) returns string {
    if !(name is "") {
        return "Hello, " + name;
    }
    return "Hello, World!";
}

Describe your problem(s)

How ever it is not the best practice to use a NOT to negate the name is "".

Describe your solution(s)

Instead, we should use if name !is "" there.

Related area

-> Compilation

Related issue(s) (optional)

No response

Suggested label(s) (optional)

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Suggested assignee(s) (optional)

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[ShammiL]

Replaced the existing generated code with the below as passing a nil value instead of an empty string is encouraged in Ballerina.

public function hello(string? name) returns string {
    if name !is () {
        return "Hello, " + name;
    }
    return "Hello, World!";
}

Fixed in #42136