A layer's selector, for a model that reads exactly one selector string.
A string is returned as it is. A flat list of strings is the pre-4.0 shape
joinSelectorList describes: it is joined, and the producer is told
once that it should emit a string. Anything else is not a selector this kind
of model can use, and gives undefined, which every such model already
treats as "no highlight".
A layer's selector, for a model that reads exactly one selector string.
A string is returned as it is. A flat list of strings is the pre-4.0 shape joinSelectorList describes: it is joined, and the producer is told once that it should emit a string. Anything else is not a selector this kind of model can use, and gives
undefined, which every such model already treats as "no highlight".