array attribute is either True or None

The `array` attribute is documented to be a boolean which indicates if a trait is an array. However, it appears that it is actually only a boolean when a trait is an array, and is otherwise `None`. Example code (using `traits` 4.6.0):

```python
from traits.api import Array, String, Int

class Obj:
a = Array()
s = String()
i = Int()

o = Obj()

print(repr(o.a.array))
print(repr(o.s.array))
print(repr(o.i.array))
```

Output:

```
True
None
None
```

This works in a case where you might do something like:

```python
if thing.array:
# do stuff
else:
# do other stuff
```

but in general it is not good practice to rely on the `False`-like nature of `None`. It would be better for the `array` attribute to always be defined, defaulting to `False` except for actual array types.