Allow array_diff() and array_intersect() with single array argument

Allow array_diff() and array_intersect() with single array argument

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.