Make syntax::expand::expand_syntax_ext return an option

[catamorphism]

Make syntax::expand::expand_syntax_ext return a Rust expr that has type option<str> where the string is non-empty, rather than an expr with type str (where the string may be empty), as per a FIXME.

[Aatch]

I don't think this is valid any more (for one that path no longer exists). Re-open if it is.

[alexcrichton]

This looks like it's still present, it just moved to syntax::ext::env, you can find via git grep 2248 as well

[emberian]

Still relevant