make sure to ignore free plugins (#10940)

This is related to https://forum.piwik.org/t/license-missing-of-a-free-glp-plugin/21674 

Some users see a license warning for a free GPL plugin. I have spent hours trying to understand how this could possibly happen over the last weeks but not sure how it happens. It must be some kind of caching issue with a certain PHP version or something. This change might not fix the actual issue but is a good check to have on top to check for invalid licenses only if it is not a free plugin.

plugins/Marketplace/Plugins/InvalidLicenses.php

@@ -189,6 +189,9 @@ class InvalidLicenses
 
         if (!empty($paidPlugins)) {
             foreach ($paidPlugins as $plugin) {
+                if (!empty($plugin['isFree'])) {
+                    continue;
+                }
                 $pluginName = $plugin['name'];
                 if ($this->isPluginActivated($pluginName)) {
                     if (empty($plugin['consumer']['license'])) {