When sha256sums-unsigned-build.txt contains an empty line, tools/dmg2mar prints a warning

When sha256sums-unsigned-build.txt contains an empty line, tools/dmg2mar prints a warning at line 98 and 99 because $filename is undef:

        my (undef, $filename) = split '  ', $line;
        chomp $filename;
        next unless $filename =~ m/^$appname_dmg-(.+)-macos_(.+)\.dmg$/;