Don't fail date validation on PHP >= 8.2

[makise-homura]

As specified here, before PHP 8.2.0, DateTime::getLastErrors() did not return false when there were no warnings or errors. Instead, it would always return the documented array structure. But on PHP 8.2.0 and newer, it actually may be false, and using array_sum() on it shall cause an error like:

[2024-02-13 19:47:40] production.ERROR: array_sum(): Argument #1 ($array) must be of type array, false given {"userId":1,"exception":"[object] (TypeError(code: 0): array_sum(): Argument #1 ($array) must be of type array, false given at /opt/CDash/app/cdash/app/Controller/Api/ResultsApi.php:84)
[stacktrace]
#0 /opt/CDash/app/cdash/app/Controller/Api/ResultsApi.php(84): array_sum()
#1 /opt/CDash/app/cdash/app/Controller/Api/ResultsApi.php(153): CDash\\Controller\\Api\\ResultsApi->validateDateString()
...

And this leads to complete unusable build results pages except the one for current date.

This PR fixes it.

[codeling]

This is the only fix required for running CDash 3.2.3/3.3.0 on PHP 8.2 as far as I can tell, thanks!
Considering that PHP 8.1 is already in "security fixes only" mode I would really love for this fix to be included in the next release!

[josephsnyder]

While I haven't been able to get the error to appear in the log as reported, this nicely accounts for the new return type. LGTM!