Description:
------------
PHP does not emit notices in switch where the only case is the default case (or where there are no cases)

This can be fixed by Zend/zend_compile.c emitting a CHECK_VAR opcode when the temporary value is IS_CV (a compiled variable such as $undefSwitch), in the cases where:
- there are no case statements
- if the only case statement is the default case.

Noticed in https://github.com/php/php-src/pull/5371#discussion_r410947575

https://bugs.php.net/bug.php?id=52426 is related, but it's from 2010 (php 5.2), and I expect opinions have changed since then, e.g. with https://wiki.php.net/rfc/engine_warnings getting approved



Test script:
---------------
<?php
ini_set('display_errors', 'stderr');
error_reporting(E_ALL);
echo $undefVar;  // This does emit a notice
switch($undefSwitch) {}  // This does not emit a notice
switch($undefSwitch2) { default: echo "default\n";}  // This does not emit a notice


Expected result:
----------------
PHP should emit an undefined variable notice for the two switch statements in the test script

Actual result:
--------------
PHP does not emit any notices for the two switch statements in the test script, in php 5.3, 5.6, 7.0, 7.4, and 8.0-dev

Apparently, it did emit notices in 5.5 - https://bugs.php.net/bug.php?id=68105 suggests that emitting notices was the expected behavior for 5.5.