methods converted to lambda-friendly can not be used with non-expression arguments

The following code fails:

```python
from math import isfinite
from mini_lambda import C

a = C(isfinite)  # define a constant function (a lambda-friendly function)
f = _(a(0) & a(0))  # raises AttributeError "NoneType has no attribute '_root_var'"
assert f(None)
```