return a pair only if they are both defined

 Moose classes (and some other classes) distinguish between an attribute
 being unset and the attribute being set to undef. Supplying a
 constructor arguments like this:
 .
     my $bob = Person->new(
        name => $name,
        age => $age,
        );
 .
 Will result in the "name" and "age" attributes possibly being set to
 undef (if the corresponding $name and $age variables are not defined),
 which may violate the Person class' type constraints.
 .
 PerlX::Maybe checks that $x and $y are both defined. If they are, it
 returns them both as a list; otherwise it returns the empty list.
 .
 This package also includes Syntax::Feature::Maybe.