MDL-77868 mod_bigbluebuttonbn: default config for welcome editable.

The property was added to the config options in c3f4123b, however no
default value was provided, meaning if it was ever unset then a PHP
type error would be triggered on the null return.

mod/bigbluebuttonbn/classes/local/config.php

@@ -110,6 +110,7 @@ class config {
             'hideuserlist_default' => false,
             'hideuserlist_editable' => true,
             'welcome_default' => '',
+            'welcome_editable' => true,
             'default_dpa_accepted' => false,
             'poll_interval' => bigbluebutton_proxy::DEFAULT_POLL_INTERVAL,
         ];
@@ -143,7 +144,7 @@ class config {
         if (isset($CFG->{'bigbluebuttonbn_' . $setting})) {
             return (string) $CFG->{'bigbluebuttonbn_' . $setting};
         }
-        return self::defaultvalue($setting);
+        return (string) self::defaultvalue($setting);
     }
 
     /**