fix(parser): handle empty strings

Closes https://github.com/angular/angular/issues/252
IMHO, it would cleaner if we had Chain with 0, 1 or many expressions and aways return chain at top level parsing. But your PR is not making it any worse, so it is ok to merge.

I checked a few other language parsers:
http://esprima.org/demo/parse.html
https://docs.python.org/2/library/ast.html 
and they both have top-level `body` expression list.