Fix two issues with ast.to_lua_source()

[quarantin]

This pull request is fixing two issues I've been encountering while using ast.to_lua_source() and for which you have issues opened:

1. When parsing a return statement that doesn't return a value, the resulting code will be: return False instead of return to_lua_source issue when returning empty value #44
2. When parsing a local assignment without a value, the resulting code will be: local some_var = instead of local some_var AST deserialization (to_lua_source) issue with assign's that have no values #37

I'm not sure if my patch is the good way to do it, but at least I tested it and it's working as expected.
Please let me know if I should change anything to get my pull-request accepted.