FeatureVariationRecord with no condition set causes error

[garretrieger]

Using the attached font file: subset.zip

fonttools varLib.instancer subset.ttf wght=100

Restricting axes: {'wght': (100, 100, 100)}
Loading variable font
Normalized limits: {'wght': (-0.751892, -0.751892, -0.751892)}
Instantiating glyf/gvar tables
Instantiating HVAR table
Instantiating FeatureVariations of GSUB table
Traceback (most recent call last):
File "/usr/local/google/home/grieger/.local/bin/fonttools", line 8, in
sys.exit(main())
^^^^^^
File "/usr/lib/python3/dist-packages/fontTools/main.py", line 30, in main
runpy.run_module(mod, run_name='main')
File "", line 229, in run_module
File "", line 88, in _run_code
File "/usr/lib/python3/dist-packages/fontTools/varLib/instancer/main.py", line 5, in
sys.exit(main())
^^^^^^
File "/usr/lib/python3/dist-packages/fontTools/varLib/instancer/init.py", line 1534, in main
instantiateVariableFont(
File "/usr/lib/python3/dist-packages/fontTools/varLib/instancer/init.py", line 1301, in instantiateVariableFont
instantiateFeatureVariations(varfont, normalizedLimits)
File "/usr/lib/python3/dist-packages/fontTools/varLib/instancer/init.py", line 844, in instantiateFeatureVariations
_instantiateFeatureVariations(
File "/usr/lib/python3/dist-packages/fontTools/varLib/instancer/init.py", line 969, in _instantiateFeatureVariations
applies, shouldKeep = _instantiateFeatureVariationRecord(
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3/dist-packages/fontTools/varLib/instancer/init.py", line 894, in _instantiateFeatureVariationRecord
for i, condition in enumerate(record.ConditionSet.ConditionTable):
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'ConditionTable'

From the specification a null ConditionSet offset is valid (https://learn.microsoft.com/en-us/typography/opentype/spec/chapter2#featurevariations-table):

"If the ConditionSet offset is 0, there is no condition set table. This is treated as the universal condition: all contexts are matched."