MDL-78937 lib: Set default value to $component property

We encountered errors on the plugin check page while upgrading from version 3.11 to 4.3.
The plugins marked with the status "To be deleted" have a null value for the $component property,
which results in the following error message:
"Deprecated: strpos(): Passing null to parameter #1 ($haystack) of type string is
deprecated in lib/classes/component.php on line 1056."
To avoid this error, we need to set an empty string as the default value for the $component property.

lib/classes/plugininfo/base.php

@@ -84,7 +84,7 @@ abstract class base {
     public $incompatible;
 
     /** @var string Name of the plugin */
-    public $component;
+    public $component = '';
 
     /**
      * Whether this plugintype supports its plugins being disabled.