MM-69340: fix NULL Type scan error in Draft.Get() (#37337)

* MM-69340: fix NULL Type scan error in Draft.Get()

draftSliceColumns() returned "Type" bare; rows created before the
v11.3 migration have Type = NULL, causing a scan error when the single-
row Get() path tried to read them into a non-nullable Go string.

Apply the same COALESCE(Type, '') fix that already existed in
GetDraftsForUser(), and add a regression test that inserts a NULL-Type
row and asserts Get() succeeds with Type == "".

* fix: apply COALESCE only in Get(), not in Upsert columns

draftSliceColumns() is used by Upsert's INSERT column list, where
COALESCE is not valid syntax. Add draftSelectColumns() which swaps in
COALESCE(Type, '') AS Type for the SELECT path only.

server/channels/store/sqlstore/draft_store.go

@@ -39,6 +39,13 @@ func draftSliceColumns() []string {
 	}
 }
 
+func draftSelectColumns() []string {
+	cols := make([]string, len(draftSliceColumns()))
+	copy(cols, draftSliceColumns())
+	cols[len(cols)-1] = "COALESCE(Type, '') AS Type"
+	return cols
+}
+
 func draftToSlice(draft *model.Draft) []any {
 	return []any{
 		draft.CreateAt,
@@ -65,7 +72,7 @@ func newSqlDraftStore(sqlStore *SqlStore, metrics einterfaces.MetricsInterface)
 
 func (s *SqlDraftStore) Get(userId, channelId, rootId string, includeDeleted bool) (*model.Draft, error) {
 	query := s.getQueryBuilder().
-		Select(draftSliceColumns()...).
+		Select(draftSelectColumns()...).
 		From("Drafts").
 		Where(sq.Eq{
 			"UserId":    userId,

server/channels/store/storetest/draft_store.go

@@ -362,6 +362,18 @@ func testGetDraft(t *testing.T, rctx request.CTX, ss store.Store) {
 		assert.Equal(t, draft2.Message, draftResp.Message)
 		assert.Equal(t, draft2.ChannelId, draftResp.ChannelId)
 	})
+
+	t.Run("get draft with NULL type", func(t *testing.T) {
+		_, err := ss.GetInternalMasterDB().Exec(
+			"UPDATE Drafts SET Type = NULL WHERE UserId = $1 AND ChannelId = $2",
+			user.Id, channel.Id,
+		)
+		require.NoError(t, err)
+
+		draftResp, err := ss.Draft().Get(user.Id, channel.Id, "", false)
+		require.NoError(t, err)
+		assert.Equal(t, "", draftResp.Type)
+	})
 }
 
 func testGetDraftsForUser(t *testing.T, rctx request.CTX, ss store.Store) {