MDL-75308 task: explicit check for tasks of disabled plugins.

The enabled state of a plugin is a tri-state value (boolean or
null). We should be consistent with elsewhere by only considering
boolean false as an actually disabled plugin.

See also cdcb53a1 for similar change in task management.

lib/classes/task/task_base.php

@@ -254,7 +254,7 @@ abstract class task_base {
             return true;
         } else {
             $plugininfo = core_plugin_manager::instance()->get_plugin_info($component);
-            return $plugininfo && $plugininfo->is_enabled();
+            return $plugininfo && ($plugininfo->is_enabled() !== false);
         }
     }
 }