Fix error report generation with null context

context may be null, which transferred as post parameter becomes
is received as empty string, but that was handled when shortening
the lines.

Signed-off-by: Maximilian Krög <maxi_kroeg@web.de>

libraries/classes/ErrorReport.php

@@ -246,12 +246,14 @@ public function send(array $report)
     private function translateStacktrace(array $stack): array
     {
         foreach ($stack as &$level) {
-            foreach ($level['context'] as &$line) {
-                if (mb_strlen($line) <= 80) {
-                    continue;
-                }
+            if (is_array($level['context'])) {
+                foreach ($level['context'] as &$line) {
+                    if (mb_strlen($line) <= 80) {
+                        continue;
+                    }
 
-                $line = mb_substr($line, 0, 75) . '//...';
+                    $line = mb_substr($line, 0, 75) . '//...';
+                }
             }
 
             [$uri, $scriptName] = $this->sanitizeUrl($level['url']);