Fix NullCallableString() when called with no or multiple arguments

NullCallbableString() doesn't work with things like clear() and move(x, y). This patch fixes this behaviour:

```python
$ cat foo.py
from blessings import Terminal
term = Terminal()
term.clear()
$ python foo.py > /dev/null 
Traceback (most recent call last):
  File "foo.py", line 4, in <module>
    term.clear()
TypeError: __call__() takes exactly 2 arguments (1 given)
```