Bad default value handling for Union

[mdickinson]

In #1522, I introduced the following bug, which broke the Pyface test suite:

Python 3.9.7 (default, Sep  1 2021, 04:00:27) 
[Clang 11.0.3 (clang-1103.0.32.62)] on darwin
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>>> from traits.api import *
>>> class A(HasTraits):
...     foo = Union(None, Int)
... 
>>> class B(A):
...     foo = 5
... 
>>> b = B()
>>> b.foo -= 1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for -=: '_Undefined' and 'int'