MDL-76356 various: avoid implicit conversion to arrays

PHP before version 8.1 automatically converted stdClass or 'false' to arrays if
function parameter expects array (for example, "reset").
PHP 8.1 shows notices in these situations

admin/roles/tests/privacy/provider_test.php

@@ -187,7 +187,7 @@ public function test_export_user_data() {
             $writer = writer::with_context($context);
             $this->assertTrue($writer->has_any_data());
             if ($context->contextlevel == CONTEXT_MODULE) {
-                if ($data = $writer->get_data($subcontextstudent)) {
+                if ($data = (array)$writer->get_data($subcontextstudent)) {
                     $this->assertEquals($user->id, reset($data)->userid);
                 }
                 if ($data = (array)$writer->get_data($subcontextrc)) {

admin/tool/policy/classes/output/page_agreedocs.php

@@ -278,7 +278,7 @@ protected function redirect_to_policies($userid, $returnurl = null) {
             $cache = \cache::make('core', 'presignup');
             $cachekey = 'tool_policy_viewedpolicies';
 
-            $viewedpolicies = $cache->get($cachekey);
+            $viewedpolicies = $cache->get($cachekey) ?: [];
             if (!empty($viewedpolicies)) {
                 // Get the list of the policies docs which the user haven't viewed during this session.
                 $pendingpolicies = array_diff($currentpolicyversionids, $viewedpolicies);

analytics/classes/calculation_info.php

@@ -99,7 +99,7 @@ public function save(\core_analytics\calculable $calculable, \core_analytics\loc
             $samplekey = self::get_sample_key($uniquesampleid);
 
             // Update the cached data adding the new indicator data.
-            $cacheddata = $cache->get($samplekey);
+            $cacheddata = $cache->get($samplekey) ?: [];
             $cacheddata[$calculableclass] = $infokeys;
             $cache->set($samplekey, $cacheddata);
         }

h5p/tests/helper_test.php

@@ -305,7 +305,9 @@ public function test_get_messages(): void {
         $this->assertTrue(empty($messages->info));
 
         // Add an some messages manually and check they are still there.
+        $messages->error = [];
         $messages->error['error1'] = 'Testing ERROR message';
+        $messages->info = [];
         $messages->info['info1'] = 'Testing INFO message';
         $messages->info['info2'] = 'Testing INFO message';
         helper::get_messages($messages, $factory);

lib/completionlib.php

@@ -1055,7 +1055,7 @@ public function get_data($cm, $wholecourse = false, $userid = 0, $unused = null)
             if (!isset($this->course->cacherev)) {
                 $this->course = get_course($this->course_id);
             }
-            if ($cacheddata = $completioncache->get($key)) {
+            if ($cacheddata = ($completioncache->get($key) ?: [])) {
                 if ($cacheddata['cacherev'] != $this->course->cacherev) {
                     // Course structure has been changed since the last caching, forget the cache.
                     $cacheddata = array();