Compare two ListValue instances always return `true`

[Pierrick Voulet]

What version of protobuf and what language are you using?
Version: php-upb-msg (protobuf2) AND protobuf 3.12.2
Language: PHP

What operating system (Linux, Windows, ...) and version?
Linux 5.2.17-1rodete3-amd64 x86_64

What runtime / compiler are you using (e.g., python version or gcc version)
PHP Extension (C implementation)

What did you do?
Run the following script:

<?php
$m1 = new ListValue();
$m1->mergeFromJsonString("[1]");

$m2 = new ListValue();
$m2->mergeFromJsonString("[2]");
print (($m1 == $m2) ? "KO" : "OK");

What did you expect to see
OK

What did you see instead?
KO

Anything else we should know about your project / environment
None.