Both of these functions are well-defined when used with a single
array argument -- rejecting this case was an artificial limitation.
This is not useful when called with explicit arguments, but removes
edge-cases when used with argument unpacking:
// OK even if $excludes is empty.
array_diff($array, ...$excludes);
// OK even if $arrays contains a single array only.
array_intersect(...$arrays);
This matches the behavior of functions like array_merge() and
array_push(), which also allow calls with no array or a single
array respectively.
Closes GH-6097.
