transitive_closure(reflexive => 0) omits isolated vertices

[michro6ok]

When computing the non-reflexive transitive closure of a directed graph, isolated vertices are missing from the result.

Test case:

#!/usr/bin/perl
use strict;
use warnings;
use Graph;

my $g = Graph->new(directed => 1);
$g->add_edge('A', 'B');
$g->add_edge('B', 'C');
$g->add_vertex('d'); # <- isolated
$g->add_vertex('e'); # <- isolated

print "graph:" . $g->stringify . "\n";
print "transitive closure:" . $g->transitive_closure(reflexive => 0)->stringify . "\n";

Output:

graph:A-B,B-C,d,e
transitive closure:A-B,A-C,B-C

Expected behavior:

graph:A-B,B-C,d,e
transitive closure:A-B,A-C,B-C,d,e

The transitive closure should include all vertices from the original graph, including isolated ones (e, f). These are currently omitted in the closure result.