Fix empty return value list producing a stray Nil

Passing an emtpy list returned by a Perl 5 function to another Perl 5
function led to the latter getting an extra argument via the
Nil -> Any -> undef chain.

Fix by replacing return; with return @retvals; (which is empty).

lib/Inline/Perl5.pm6

@@ -434,9 +434,10 @@ method !setup_arguments(@args) {
 method !unpack_return_values($av) {
     my int32 $av_len = p5_av_top_index($!p5, $av);
 
+    my @retvals;
     if $av_len == -1 {
         p5_sv_refcnt_dec($!p5, $av);
-        return;
+        return @retvals; # avoid returning Nil when there are no return values
     }
 
     if $av_len == 0 {
@@ -445,7 +446,6 @@ method !unpack_return_values($av) {
         return $retval;
     }
 
-    my @retvals;
     loop (my int32 $i = 0; $i <= $av_len; $i = $i + 1) {
         @retvals.push(self.p5_to_p6(p5_av_fetch($!p5, $av, $i)));
     }

t/invoke.t

@@ -16,10 +16,28 @@ $p5.run: q:heredoc/PERL5/;
     sub push {
         return 'pushed';
     }
+
+    sub nothing {
+        return;
+    }
+
+    sub empty_hash {
+        return {};
+    }
+
+    sub count_args {
+        return scalar @_;
+    }
+
     PERL5
 
 my $foo = $p5.invoke('Foo', 'new');
 
 is($foo.push, 'pushed');
+my @a = $foo.nothing;
+is($foo.count_args($foo.nothing), 1);
+is($foo.count_args($foo.empty_hash), 2);
 
 done;
+
+# vim: ft=perl6