For unbounded conditions, the previous code expects "minimum" or
"maximum" to be entirely absent, whereas actually they will be
consistently present with one having a value of None.
This means that math.inf and -math.inf are never substituted in for the
absent bound, and a crash occurs when the None value propagates.
This commit corrects the behaviour by checking for a value of None,
instead of checking for the presence of the keys, bringing the
behaviour inline with the rest of the library.
fef9e9a071