`default_value_if` with `IsPresent` doesn't ignore defaults

    /// ```rust
    /// # use clap_builder as clap;
    /// # use clap::{Command, Arg, ArgAction};
    /// # use clap::builder::{ArgPredicate};
    /// let m = Command::new("prog")
    ///     .arg(Arg::new("flag")
    ///         .long("flag")
    ///         .action(ArgAction::SetTrue))
    ///     .arg(Arg::new("other")
    ///         .long("other")
    ///         .default_value_if("flag", ArgPredicate::IsPresent, Some("default")))
    ///     .get_matches_from(vec![
    ///         "prog"
    ///     ]);
    ///
    /// assert_eq!(m.get_one::<String>("other"), None);
    /// ```