Fix RemoveDeadIfBlockRector dropping else block when merging empty if with elseif

[TomasVotruba]

Fixes rectorphp/rector#9776

Problem

When an if block has an empty body but is followed by an elseif (with statements) and an else block, RemoveDeadIfBlockRector merges the negated if condition into the first elseif, but silently drops the else block.

Input

if ($qty < 1) {
} elseif (!$cond) {
    $total = $qty * $price1;
} else {
    $total = $qty * $price2;
}

Actual (buggy)

if ($qty >= 1 && !$cond) {
    $total = $qty * $price1;
}

Expected

if ($qty >= 1 && !$cond) {
    $total = $qty * $price1;
} else {
    $total = $qty * $price2;
}

Fix

When constructing the new merged If_ node, carry over the original else block (alongside the already-handled remaining elseifs).

A fixture covering the issue's reproduction has been added.

[TomasVotruba]

LGTM 👍