hval: to_filename: return `undef' instead of empty string

Returning an empty string for a filename makes no sense,
so instead return `undef' so the caller can setup a fallback
using the "//" operator.

This fixes uninitialized variable warnings because split()
on an empty string returns `undef', which caused to_filename
to warn on s// and tr// ops.

1 files changed, 5 insertions, 3 deletions

diff --git a/t/hval.t b/t/hval.t
index 38605c6f..e80a02ff 100644
--- a/t/hval.t
+++ b/t/hval.t
@@ -47,15 +47,17 @@ EOF
 
 is($html, $exp, 'only obfuscated relevant addresses');
 
-is('foo-bar', PublicInbox::Hval::to_filename('foo bar  '),
+is(PublicInbox::Hval::to_filename('foo bar  '), 'foo-bar',
         'to_filename has no trailing -');
 
-is('foo-bar', PublicInbox::Hval::to_filename("foo   bar\nanother line\n"),
+is(PublicInbox::Hval::to_filename("foo   bar\nanother line\n"), 'foo-bar',
         'to_filename has no repeated -, and nothing past LF');
 
-is('foo.bar', PublicInbox::Hval::to_filename("foo....bar"),
+is(PublicInbox::Hval::to_filename("foo....bar"), 'foo.bar',
         'to_filename squeezes -');
 
+is(PublicInbox::Hval::to_filename(''), undef, 'empty string returns undef');
+
 my $s = "\0\x07\n";
 PublicInbox::Hval::src_escape($s);
 is($s, "\\0\\a\n", 'src_escape works as intended');