[css-values-5] Require two or more `random-item()`s

[cdoublev]

I am not sure whether the current requirement of one or more random items in random-item() is intentional or if it should be two or more:

- <random-item()> = random-item( <random-caching-options> ';' <declaration-value> [ ';' <declaration-value> ]* )
+ <random-item()> = random-item( <random-caching-options> ';' <declaration-value> [ ';' <declaration-value> ]+ )

Alternatively:

<random-item()> = random-item( <random-caching-options> [ ';' <declaration-value> ]{2,} )

[brandonmcconnell]

@cdoublev I think one or more would be better, so when we spread() values arbitrarily (re #8391), they can contain any non-zero number of values.

[tabatkins]

The current restriction is intentional. It's perfectly well-defined to select from a list of length 1, and consistent with the behavior of lengths >1. (As opposed to an empty list, which would have to have some novel behavior.)

[cdoublev]

Thank you both for your clarification.