fix(model): prevent overwriting primary key on save

src/Libs/Model.php

@@ -480,12 +480,8 @@ class Model
 
         $set = [];
         foreach ($atts as $key => $value) {
-            if ($value !== null) {
             $set[] = "$key=:$key";
             static::$dbQueryVariables[':' . $key] = $value;
-            } else {
-                $set[] = "$key=NULL";
-            }
         }
 
         $table = static::table();
@@ -511,19 +507,17 @@ class Model
         static::$dbQueryVariables = [];
 
         foreach ($atts as $key => $value) {
-            if (isset($value)) {
             $into[] = "$key";
             $values[] = ":$key";
             static::$dbQueryVariables[":$key"] = $value;
         }
-        }
 
         $table = static::table();
         $sql = "INSERT INTO $table (" . join(', ', $into) . ") VALUES (" . join(', ', $values) . ")";
         static::query($sql);
 
         $pk = static::$dbPrimaryKey;
-        $this->$pk = $db->lastInsertId();
+        $this->$pk ??= $db->lastInsertId();
         $this->markAsSaved();
     }