MDL-66392 analytics: make model output directory default to empty.

Falling back to path within $CFG->dataroot/models.

analytics/classes/model.php

@@ -1492,6 +1492,17 @@ class model {
             $prediction->get_prediction_data()->contextid, $prediction->get_sample_data());
     }
 
+    /**
+     * Returns the default output directory for prediction processors
+     *
+     * @return string
+     */
+    public static function default_output_dir(): string {
+        global $CFG;
+
+        return $CFG->dataroot . DIRECTORY_SEPARATOR . 'models';
+    }
+
     /**
      * Returns the output directory for prediction processors.
      *
@@ -1506,8 +1517,6 @@ class model {
      * @return string
      */
     public function get_output_dir($subdirs = array(), $onlymodelid = false) {
-        global $CFG;
-
         $subdirstr = '';
         foreach ($subdirs as $subdir) {
             $subdirstr .= DIRECTORY_SEPARATOR . $subdir;
@@ -1516,7 +1525,7 @@ class model {
         $outputdir = get_config('analytics', 'modeloutputdir');
         if (empty($outputdir)) {
             // Apply default value.
-            $outputdir = rtrim($CFG->dataroot, '/') . DIRECTORY_SEPARATOR . 'models';
+            $outputdir = self::default_output_dir();
         }
 
         // Append model id.