no argument checking is done on a lambda with no parameters

_CRuby_:
```ruby
(lambda {}).call(1) #=> ArgumentError: wrong number of arguments (given 1, expected 0)
(lambda {||}).call(1) #=> ArgumentError: wrong number of arguments (given 1, expected 0)
```
_mruby_:
```ruby
(lambda {}).call(1) #=> nil
(lambda {||}).call(1) #=> nil
```
Tested on 26475d0.