`set_port`, `set_username` & `set_password` should check for a value before returning `Err(())`

The methods set_port, set_username and set_password all fail early if called when the url cannot have a port/username/password:

    pub fn set_port(&mut self, mut port: Option<u16>) -> Result<(), ()> {
        // has_host implies !cannot_be_a_base
        if !self.has_host() || self.host() == Some(Host::Domain("")) || self.scheme() == "file" {
            return Err(());
        }

    pub fn set_username(&mut self, username: &str) -> Result<(), ()> {
        // has_host implies !cannot_be_a_base
        if !self.has_host() || self.host() == Some(Host::Domain("")) || self.scheme() == "file" {
            return Err(());
        }

    pub fn set_password(&mut self, password: Option<&str>) -> Result<(), ()> {
        // has_host implies !cannot_be_a_base
        if !self.has_host() || self.host() == Some(Host::Domain("")) || self.scheme() == "file" {
            return Err(());
        }

I believe they should first check for the existence of a value (or empty string, in the case of set_username) before returning an error.

    pub fn set_username(&mut self, username: &str) -> Result<(), ()> {
        // has_host implies !cannot_be_a_base
        if !self.has_host() || self.host() == Some(Host::Domain("")) || self.scheme() == "file" {
+           if username.is_empty() {
+                return Ok(());
+           }
            return Err(());
        }

    pub fn set_password(&mut self, password: Option<&str>) -> Result<(), ()> {
        // has_host implies !cannot_be_a_base
        if !self.has_host() || self.host() == Some(Host::Domain("")) || self.scheme() == "file" {
+           if password.is_none() || password == Some("") {
+                return Ok(());
+           }
            return Err(());
        }

Start with the implementations of set_port, set_username, and set_password shown in the issue. Check the existing behavior for URLs that cannot have ports, usernames, or passwords, then add coverage for absent or empty values. Done means empty values are accepted where specified while nonempty values still return Err(()).