This PR fixes a bug with typed process outputs. Typed outputs can be arbitrary expressions, including function calls:

nextflow.enable.types = true

process ADD {
    input:
    x: Integer
    y: Integer

    output:
    add(x, y)

    exec:
    true
}

def add(x, y) {
    return x + y
}

However, this currently fails. Apparently, both variable references and function calls are routed through TaskOutputResolver's get() method, which currently expects only variables

This PR changes get() to return null instead of failing, allowing the method dispatch to fall back to invokeMethod()

The try-catch was inherited from legacy processes, but it is not needed here because an undefined variable in a typed process will always be caught at compile-time