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;
         }