How can I UPSERT a row with serial identity in PostgreSQL using ADO.NET?

Given a table like:

CREATE TABLE myTable (
 identity SERIAL PRIMARY KEY,
 foo VARCHAR(32) NOT NULL
)

I would like to perform an UPSERT on that table using ADO.NET, my POCO in C# looks like:

public class myEntity {
 //When null the entity has never been stored in the database
 public int? Identity { get; set; }
 public string Foo { get; set; }
}

I came up with the following query to use with a DbCommand

INSERT INTO myTable (identity, foo) VALUES (@identity, @foo) 
ON CONFLICT (identity) DO UPDATE SET
foo = @foo
RETURNING identity;

The DbCommand is initialized as follows

using (var dbCommand = new NpgsqlCommand(INSERT_QUERY, dbConnection))
{
  dbCommand.Parameters.AddWithValue("@identity", oneOfMyEntities.Identity);
  dbCommand.Parameters.AddWithValue("@foo", oneOfMyEntities.Foo);

  int idReturned = (int)await dbCommand.ExecuteScalar();

  if (!oneOfMyEntity.Identity.HasValue) oneOfMyEntity.Identity = idReturned;
}

What I would like to achieve is that when an instance of myEntity has the property Identity set to null a new row is inserted into the database with the identity generated from the sequence, otherwise an update is performed.

The problem is that in PostgreSQL the only way to "force" the use of the next value in the sequence is to either omit the identity column from the INSERT query or to use DEFAULT. I can't simply put the literal "DEFAULT" in the dbCommand parameter @identity because, as far as I know, that would be interpreted as a string value a not the keyword DEFAULT.

I've tried to generate the query text at runtime so that if the entity has not an identity I replace the @identity parameter with DEFAULT, but this doesn't look good to me because I could simply remove ON CONFLICT, have two queries (one INSERT and one UPDATE) and execute an INSERT or an UPDATE depending on whether the entity identity is null or not thus not taking advantage of the ON CONFLICT clause.

How can I write this logic so that when an entity as a null identity I can somehow "force" PostgreSQL to use the next value in the sequence without resorting to generating the query text at runtime (and replacing the @identity parameter with DEFAULT) or using an UPDATE or an INSERT separately depending on the case ?