Retrieve the user information component of the URI.

If no user information is present, this method MUST return an empty
string.

If a user is present in the URI, this will return that value;
additionally, if the password is also present, it will be appended to the
user value, with a colon (":") separating the values.

NOTE that be default, the password, if available, will NOT be shown
as a security measure as discussed in RFC 3986, Section 7.5. If you know
the password is not a security issue, you can force it to be shown
with $this->showPassword();

The trailing "@" character is not part of the user information and MUST
NOT be added.

@return string|null The URI user information, in "username[:password]" format.