MDL-80516 core: more robust plugin update component checking.

Occurs when a plugin in incorrectly removed (via file system only,
rather than following proper uninstall process), and also contains
it's own sub-plugins.

In the above scenario, the update would request and receive back a
response for a plugin whose name was comprised of an empty string,
subsequently causing errors when processing said response.

lib/classes/plugin_manager.php

@@ -1550,7 +1550,7 @@ class core_plugin_manager {
 
         $provider = \core\update\checker::instance();
 
-        if (!$provider->enabled() or during_initial_install()) {
+        if (!$provider->enabled() || $component === '' || during_initial_install()) {
             return null;
         }
 

lib/classes/update/checker.php

@@ -440,8 +440,11 @@ class checker {
 
         $pluginman = \core_plugin_manager::instance();
         foreach ($pluginman->get_plugins() as $type => $plugins) {
+            // Iterate over installed plugins and determine which are non-standard and eligible for update checks. Note that we
+            // disregard empty component names here, to ensure we only request valid data from the update site (in the case of an
+            // improperly removed plugin containing sub-plugins, we would get an empty value here for each sub-plugin).
             foreach ($plugins as $plugin) {
-                if (!$plugin->is_standard()) {
+                if ($plugin->component !== '' && !$plugin->is_standard()) {
                     $this->currentplugins[$plugin->component] = $plugin->versiondisk;
                 }
             }