[PATCH] t/lei-convert: fix uninitialized variable w/o pigz

[Eric Wong]

`backtick` captures return `undef' when a command is missing

Fixes: 5df0446abcca (lei: don't gzip --rsyncable by default for mbox*)
---
 t/lei-convert.t | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/t/lei-convert.t b/t/lei-convert.t
index d75110cb..84b57f81 100644
--- a/t/lei-convert.t
+++ b/t/lei-convert.t
@@ -132,7 +132,7 @@ test_lei({ tmpdir => $tmpdir }, sub {
        SKIP: {
                my $ok;
                for my $x (($ENV{GZIP}//''), qw(pigz gzip)) {
-                       $x && `$x -h 2>&1` =~ /--rsyncable\b/s or next;
+                       $x && (`$x -h 2>&1`//'') =~ /--rsyncable\b/s or next;
                        $ok = $x;
                        last;
                }