MDL-75534 Fix usermodified in very old forum discussions

Very early Moodle versions used to leave the field empty. Recent Moodle
versions expect that the value always links to an existing user account.

mod/forum/db/upgrade.php

@@ -173,5 +173,12 @@ function xmldb_forum_upgrade($oldversion) {
         upgrade_mod_savepoint(true, 2022072900, 'forum');
     }
 
+    if ($oldversion < 2022081900) {
+        // Some very old discussions from early Moodle versions may have the usermodified set to zero.
+        $DB->execute("UPDATE {forum_discussions} SET usermodified = userid WHERE usermodified = 0");
+
+        upgrade_mod_savepoint(true, 2022081900, 'forum');
+    }
+
     return true;
 }

mod/forum/version.php

@@ -24,6 +24,6 @@
 
 defined('MOODLE_INTERNAL') || die();
 
-$plugin->version   = 2022072900;       // The current module version (Date: YYYYMMDDXX).
+$plugin->version   = 2022081900;       // The current module version (Date: YYYYMMDDXX).
 $plugin->requires  = 2022041200;       // Requires this Moodle version.
 $plugin->component = 'mod_forum';      // Full name of the plugin (used for diagnostics)