Npgsql JSONB to POCO mapping produces an empty `open` stmt in `Migration/*Snapshot.fs`

[nodakai]

Split from #160 (comment)

Describe the bug

When utilizing Npgsql's JSONB to POCO mapping, Migrations/*Snapshot.fs gets emitted with an empty, malformed open statement:

open System
open System.Text.Json
open 
open Microsoft.EntityFrameworkCore
open Microsoft.EntityFrameworkCore.Infrastructure
open Microsoft.EntityFrameworkCore.Metadata
open Microsoft.EntityFrameworkCore.Migrations
open Microsoft.EntityFrameworkCore.Storage.ValueConversion
open Npgsql.EntityFrameworkCore.PostgreSQL.Metadata

To Reproduce
I think this mode of utilizing JSON columns, which I started exploring recently, produces an empty open statement
https://www.npgsql.org/efcore/mapping/json.html#poco-mapping

public class SomeEntity
{
    public int Id { get; set; }
    [Column(TypeName = "jsonb")]
    public Customer Customer { get; set; }
}

public class Customer    // Mapped to a JSON column in the table
{
    public string Name { get; set; }
    public int Age { get; set; }
    public Order[] Orders { get; set; }
}

That is, with Npgsql, using [Column(TypeName = "jsonb")] enables an arbitrary type (e.g., Customer) to be included as part of an entity (e.g., SomeEntity), without the need to register the former as an entity using DbSet. I can imagine this being a tricky corner case for the migration builder.

Expected behavior

open System
open System.Text.Json
open Microsoft.EntityFrameworkCore
open Microsoft.EntityFrameworkCore.Infrastructure
open Microsoft.EntityFrameworkCore.Metadata
open Microsoft.EntityFrameworkCore.Migrations
open Microsoft.EntityFrameworkCore.Storage.ValueConversion
open Npgsql.EntityFrameworkCore.PostgreSQL.Metadata

Desktop (please complete the following information):

dotnet list package                                                               
Project 'EfPostgresPotionalFk' has the following package references
  [net7.0]: 
  Top-level Package                            Requested   Resolved
  > EntityFrameworkCore.FSharp                 6.0.7       6.0.7   
  > FSharp.Core                                7.0.402     7.0.402 
  > FSharpPlus                                 1.5.0       1.5.0   
  > Microsoft.EntityFrameworkCore.Design       6.0.25      6.0.25  
  > Npgsql.EntityFrameworkCore.PostgreSQL      6.0.22      6.0.22  

Additional context

If I just remove the empty open statement, the project seems to compile and work as expected, so this issue isn't a real show stopper for me