Allow negated assignment in else

[muglug]

TypeTest::testNegatedAssignmentInIf():

if (!($row = (rand(0, 10) ? [5] : null))) {
    // do nothing
}  else {
    echo $row[0];
}

This code emits a MixedArrayOffset issue because the type the if condition evaluates to empty, and negating empty produces mixed.