Fix updated at being set in SoftDeletes (#21286)

Refs #21178

Author: segadora

src/Illuminate/Database/Eloquent/SoftDeletes.php

@@ -66,7 +66,7 @@ protected function runSoftDelete()
 
         $this->{$this->getDeletedAtColumn()} = $time;
 
-        if ($this->timestamps) {
+        if ($this->timestamps && ! is_null($this->getUpdatedAtColumn())) {
             $this->{$this->getUpdatedAtColumn()} = $time;
 
             $columns[$this->getUpdatedAtColumn()] = $this->fromDateTime($time);