Test that DS5 splitting code handles unbounded conditions gracefully

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.

Tests/designspaceLib/split_test.py

@@ -1,9 +1,16 @@
+import math
 import shutil
 from pathlib import Path
 
 import pytest
 from fontTools.designspaceLib import DesignSpaceDocument
-from fontTools.designspaceLib.split import splitInterpolable, splitVariableFonts, convert5to4
+from fontTools.designspaceLib.split import (
+    _conditionSetFrom,
+    convert5to4,
+    splitInterpolable,
+    splitVariableFonts,
+)
+from fontTools.designspaceLib.types import ConditionSet, Range
 
 from .fixtures import datadir
 
@@ -148,3 +155,47 @@ def test_convert5to4(datadir, tmpdir, test_ds, expected_vfs):
             assert data_out.read_text(encoding="utf-8") == temp_out.read_text(
                 encoding="utf-8"
             )
+
+
+@pytest.mark.parametrize(
+    ["unbounded_condition"],
+    [
+        ({"name": "Weight", "minimum": 500, "maximum": None},),
+        ({"name": "Weight", "minimum": None, "maximum": 500},),
+    ],
+)
+def test_optional_min_max(unbounded_condition):
+    """Check that split functions can handle conditions that are partially
+    unbounded without tripping over the None values."""
+    doc = DesignSpaceDocument()
+
+    doc.addAxisDescriptor(
+        name="Weight", tag="wght", minimum=400, maximum=1000, default=400
+    )
+
+    doc.addRuleDescriptor(
+        name="unbounded",
+        conditionSets=[[unbounded_condition]],
+    )
+
+    assert len(list(splitInterpolable(doc))) == 1
+    assert len(list(splitVariableFonts(doc))) == 1
+
+
+@pytest.mark.parametrize(
+    ["condition", "expected_set"],
+    [
+        (
+            {"name": "axis", "minimum": 0.5, "maximum": None},
+            {"axis": Range(minimum=0.5, maximum=math.inf)},
+        ),
+        (
+            {"name": "axis", "minimum": None, "maximum": 0.5},
+            {"axis": Range(minimum=-math.inf, maximum=0.5)},
+        ),
+    ],
+)
+def test_optional_min_max_internal(condition, expected_set: ConditionSet):
+    """Check that split's internal helper functions produce the correct output
+    for conditions that are partially unbounded."""
+    assert _conditionSetFrom([condition]) == expected_set