As we make an effort to support conditions with "minimum" or "maximum"
absent, as well as with "minimum" or "maximum" None, this commit
confirms that the split functions can handle these to some degree also.
Conditions with unbounded values have a "minimum" or "maximum" value of
None.
These tests check that:
a) The public-facing split functions can receive None values without
crashing; and
b) That their internal helper functions correctly translate the None
values to math.inf and -math.inf to express them.
These tests are expected to fail, indicating where a fix is required.
