Fix handling of unbounded conditions

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.

Lib/fontTools/designspaceLib/split.py

@@ -352,9 +352,10 @@ def _extractSubSpace(
 def _conditionSetFrom(conditionSet: List[Dict[str, Any]]) -> ConditionSet:
     c: Dict[str, Range] = {}
     for condition in conditionSet:
+        minimum, maximum = condition["minimum"], condition["maximum"]
         c[condition["name"]] = Range(
-            condition.get("minimum", -math.inf),
-            condition.get("maximum", math.inf),
+            minimum if minimum is not None else -math.inf,
+            maximum if maximum is not None else math.inf,
         )
     return c