fix: suppress exec/shell_exec warnings and guard null references

DaemonTrait: added @ to shell_exec calls that may fail on restricted
systems (e.g. when /proc is unavailable).

ProcessManager::isNginxRunning(): suppressed exec warning when pgrep
is not available.

Database::query(): guarded explode on debugString output to prevent
undefined offset when Sent SQL section is missing.

UpdateCronJob: added null check for $gitRelease to prevent fatal error
when GitHubReleases service fails to initialize.

src/core/Process/ProcessManager.php

@@ -475,7 +475,7 @@ class ProcessManager {
      */
     public static function isNginxRunning() {
         $rOutput = [];
-        exec('pgrep -u xc_vm -a 2>/dev/null', $rOutput);
+        @exec('pgrep -u xc_vm -a 2>/dev/null', $rOutput);
         foreach ($rOutput as $rProcess) {
             if (preg_match('/nginx:\s+master/', $rProcess)) {
                 return true;