empty list element not flattened

[coke]

Causes test failure in S04-statement-modifiers/if.t, e.g.:

        my $answer = 0;
        my @x = 41, (42 if $answer), 43;
        my @y = 41, ($answer ?? 42 !! ()), 43;
        my @z = 41, 43;
        is @y, @z, "sanity check";
        is @x, @y, "if expr on false cond";

Last test fails, as 41,(),43 != 41,43.

Additional failures in:

S06-signature/unpack-array.t
S04-statement-modifiers/unless.t
integration/99problems-11-to-20.t