Add test for owned collection mapped to JSON

Addresses review feedback: adds a test covering the owned (non-complex)
collection case mapped to JSON. Owned collections are always nullable
(IsUnique is false), so the default value is null rather than [].

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: roji

test/EFCore.Relational.Tests/Migrations/Internal/MigrationsModelDifferTest.cs

@@ -10053,6 +10053,55 @@ public virtual void Add_complex_collection_mapped_to_json_uses_empty_array_as_de
                 Assert.IsType<DropColumnOperation>(downOps[0]);
             });
 
+#pragma warning disable EF8001 // Owned JSON entities are obsolete
+    [ConditionalFact]
+    public virtual void Add_owned_collection_mapped_to_json_has_nullable_column()
+        => Execute(
+            _ => { },
+            source =>
+            {
+                source.Entity(
+                    "Entity", e =>
+                    {
+                        e.Property<int>("Id").ValueGeneratedOnAdd();
+                        e.HasKey("Id");
+                    });
+            },
+            target =>
+            {
+                target.Entity(
+                    "Entity", e =>
+                    {
+                        e.Property<int>("Id").ValueGeneratedOnAdd();
+                        e.HasKey("Id");
+
+                        e.OwnsMany(
+                            "Owned", "json_collection", o =>
+                            {
+                                o.ToJson();
+                                o.Property<string>("Value");
+                                o.Property<DateTime>("Date");
+                            });
+                    });
+            },
+            upOps =>
+            {
+                Assert.Equal(1, upOps.Count);
+
+                var operation = Assert.IsType<AddColumnOperation>(upOps[0]);
+                Assert.Equal("Entity", operation.Table);
+                Assert.Equal("json_collection", operation.Name);
+                // Owned collections are always nullable (IsUnique is false), so no default value
+                Assert.True(operation.IsNullable);
+                Assert.Null(operation.DefaultValue);
+            },
+            downOps =>
+            {
+                Assert.Equal(1, downOps.Count);
+                Assert.IsType<DropColumnOperation>(downOps[0]);
+            });
+#pragma warning restore EF8001 // Owned JSON entities are obsolete
+
     [ConditionalFact]
     public virtual void Noop_on_complex_properties()
         => Execute(