Don't crash when there are only base-mark kerns

[simoncozens]

ufo2ft's kernFeatureWriter splits base-to-base and mark-to-base kerns like this:

ufo2ft/Lib/ufo2ft/featureWriters/kernFeatureWriter.py

Lines 449 to 452 in db8302b

	basePairs, markPairs = self._splitBaseAndMarkPairs(pairs, marks)
	lookups["LTR"] = [self._makeKerningLookup("kern_ltr", basePairs)]
	if markPairs:
	lookups["LTR"].append(

There's a horrible corner case (which I've just hit!) if the only kerns in the font are between bases and marks: basePairs is empty and the assertion here fails:

ufo2ft/Lib/ufo2ft/featureWriters/kernFeatureWriter.py

Lines 371 to 374 in db8302b

	def _makeKerningLookup(
	self, name, pairs, exclude=None, rtl=False, ignoreMarks=True
	):
	assert pairs

This fixes that.