If not having a value has a meaning in your program (e.g. an optional value), you should use None. That’s its purpose anyway.
If the value must be provided by the caller of __init__, I would recommend not to initialize it.
If “” makes sense as a default value, use it.
In Python the type is deduced from the usage. Hence, you can change the type by just assigning a value of another type.
>>> x = None
>>> print type(x)
<type 'NoneType'>
>>> x = "text"
>>> print type(x)
<type 'str'>
>>> x = 42
>>> print type(x)
<type 'int'>