Some areas of the library check for both representations, and so doing
this here too means we are less likely to break existing code.
Despite this, flexibility introduces ambiguity, and so if typing gives
us confidence that such an input is unlikely, we could re-review this;
conditions with missing keys are not safe to use across the entire code-
base.
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.
