Update User model to allow null values in username, email, and phoneNumber; adjust EditProfile view accordingly

The User model has been updated to allow null values in the username, email, and phoneNumber fields. The corresponding constructor has been updated

lib/models/user.dart

@@ -43,11 +43,11 @@ class User {
   }) {
     return User(
       id: id ?? this.id,
-      username: this.username,
+      username: username ?? this.username,
       phoneNumber: phoneNumber ?? this.phoneNumber,
       photo: photo,
       fullName: fullName ?? this.fullName,
-      email: email,
+      email: email ?? this.email,
     );
   }
 }

lib/views/profile/edit_profile/edit_profile.dart

@@ -147,7 +147,7 @@ class _EditProfileState extends State<EditProfile> {
             Positioned(
               left: 20,
               right: 20,
-              bottom: MediaQuery.of(context).viewInsets.bottom + 20,
+              bottom: 20,
               child: DidvanButton(
                 key: UniqueKey(),
                 title: 'ذخیره تغییرات',