Fix case that input is '0'

[syohex]

If input is only string '0', then loop condition is false and loop body is not evaluated. This loop should test whether $_ is empty string, not $_ is false.

Output of #111 is

% perl -MText::Xslate -E '$t=Text::Xslate->new; $t->render_string("1") eq "1" ? say "OK" : say "ERR"'
OK
% perl -MText::Xslate -E '$t=Text::Xslate->new; $t->render_string("0") eq "0" ? say "OK" : say "ERR"'
OK

This fix is related to #111, #112.

Please see this patch.

[gfx]

Thanks!