Empty arrays in isPartOf property

[remibetin]

Context: #314 (comment)

At multiple times in equal-access.json, the isPartOf property contains a WCAG success criterion value AND an empty array.

Example:

wcag-act-rules/content-assets/wcag-act-rules/earl/equal-access.json

Lines 3871 to 3874 in cb40747

	"isPartOf": [
	"WCAG2:non-text-content",
	[]
	]

I understand from EARL Reporting Format – Test Criterion that isPartOf contains "strings, listing the WCAG success criteria that fail when the rule or procedure fails".

What is the purpose of an empty array in this context?