Replace db count query in user signup process (#18072)

store/storetest/mocks/UserStore.go

@@ -1038,6 +1038,27 @@ func (_m *UserStore) InvalidateProfilesInChannelCacheByUser(userID string) {
 	_m.Called(userID)
 }
 
+// IsEmpty provides a mock function with given fields:
+func (_m *UserStore) IsEmpty() (bool, error) {
+	ret := _m.Called()
+
+	var r0 bool
+	if rf, ok := ret.Get(0).(func() bool); ok {
+		r0 = rf()
+	} else {
+		r0 = ret.Get(0).(bool)
+	}
+
+	var r1 error
+	if rf, ok := ret.Get(1).(func() error); ok {
+		r1 = rf()
+	} else {
+		r1 = ret.Error(1)
+	}
+
+	return r0, r1
+}
+
 // PermanentDelete provides a mock function with given fields: userID
 func (_m *UserStore) PermanentDelete(userID string) error {
 	ret := _m.Called(userID)

store/storetest/user_store.go

@@ -36,6 +36,7 @@ func TestUserStore(t *testing.T, ss store.Store, s SqlStore) {
 		require.NoError(t, err, "failed cleaning up test user %s", u.Username)
 	}
 
+	t.Run("IsEmpty", func(t *testing.T) { testIsEmpty(t, ss) })
 	t.Run("Count", func(t *testing.T) { testCount(t, ss) })
 	t.Run("AnalyticsActiveCount", func(t *testing.T) { testUserStoreAnalyticsActiveCount(t, ss, s) })
 	t.Run("AnalyticsActiveCountForPeriod", func(t *testing.T) { testUserStoreAnalyticsActiveCountForPeriod(t, ss, s) })
@@ -5711,3 +5712,28 @@ func testGetKnownUsers(t *testing.T, ss store.Store) {
 		assert.ElementsMatch(t, userIds, []string{u2.Id, u3.Id})
 	})
 }
+
+func testIsEmpty(t *testing.T, ss store.Store) {
+	ok, err := ss.User().IsEmpty()
+	require.NoError(t, err)
+	require.True(t, ok)
+
+	u := model.User{
+		Email:    MakeEmail(),
+		Username: model.NewId(),
+	}
+
+	_, err = ss.User().Save(&u)
+	require.NoError(t, err)
+
+	ok, err = ss.User().IsEmpty()
+	require.NoError(t, err)
+	require.False(t, ok)
+
+	err = ss.User().PermanentDelete(u.Id)
+	require.NoError(t, err)
+
+	ok, err = ss.User().IsEmpty()
+	require.NoError(t, err)
+	require.True(t, ok)
+}