Function default 

pub async fn default<P, W, E>(
    strict: bool,
    param: &P,
    default: Option<W>,
    env: &mut E,
    cfg: TildeExpansion,
) -> Result<Fields<W::EvalResult>, W::Error>
where
    P: ?Sized + ParamEval<E, EvalResult = W::EvalResult>,
    W: WordEval<E>,
    E: ?Sized,

Evaluate a parameter or use a default value if it is empty.

First, param will be evaluated and returned as is as long as the result is non-empty, or if the result is defined-but-empty and strict = false.

Otherwise, default will be evaluated using cfg and that response yielded.

Note: field splitting will neither be done on the parameter, nor the default word.