We've had an accessibility error reported by the Spanish "Portal
Administración electrónica" (PAe). While I can't find any accessibility
rule saying empty lists should be avoided, it looks like some screen
readers report finding lists with 0 items, which is annoying.
We could also do it with CSS using `ul:empty { display: none}`. However,
at the time of writing no browser supports this rule when the tag
contains whitespace.
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