Texy::outdent() - Fix for invalid argument

After this commmit* it start creating warning "Invalid argument supplied for foreach()".
Problem is when $s is empty string because Regexp::match(..) returns NULL instead array().

*) https://github.com/dg/texy/commit/75ebbf9aa27f7e79ad3ba368ad536328ebbd2744

I try this fix for me & it works. If you want (or you have) better sollution, just delete my pull ;)