Resolve failing unit tests in Moodle 4.3

[ScottVerbeek]

This merge request includes the fixes for issues:

• Failing unit tests in Moodle 4.3 #81
• Non-PostgreSQL compatible SQL #3
• Unit test failing: Error: [] operator not supported for strings #71

Here is the run for the testsuite block_quickmail_testsuite on Moodle 4.3.

root@b254dc51a88f:/var/www/vanilla-403# vendor/bin/phpunit --testsuite=block_quickmail_testsuite 
Moodle 4.3+ (Build: 20231201), a418e9e53aef54f624cec25c48a4b2bfe7bd4524
Php: 8.1.24, pgsql: 14.0 (Debian 14.0-1.pgdg110+1), OS: Linux 6.4.12-060412-generic x86_64
PHPUnit 9.5.28 by Sebastian Bergmann and contributors.

...............................................................  63 / 253 ( 24%)
............................................................... 126 / 253 ( 49%)
............................................................... 189 / 253 ( 74%)
............................................................... 252 / 253 ( 99%)
.                                                               253 / 253 (100%)

Time: 00:48.855, Memory: 90.50 MB

OK (253 tests, 1013 assertions)