Fix LDAP sync when Username Attribute is empty (#25278)

Fix #21072

![image](96b30beb-7f88-4a60-baae-2e5ad8049555)

Username Attribute is not a required item when creating an
authentication source. If Username Attribute is empty, the username
value of LDAP user cannot be read, so all users from LDAP will be marked
as inactive by mistake when synchronizing external users.

This PR improves the sync logic, if username is empty, the email address
will be used to find user.

tests/integration/auth_ldap_test.go

@@ -268,6 +268,57 @@ func TestLDAPUserSync(t *testing.T) {
 	}
 }
 
+func TestLDAPUserSyncWithEmptyUsernameAttribute(t *testing.T) {
+	if skipLDAPTests() {
+		t.Skip()
+		return
+	}
+	defer tests.PrepareTestEnv(t)()
+
+	session := loginUser(t, "user1")
+	csrf := GetCSRF(t, session, "/admin/auths/new")
+	payload := buildAuthSourceLDAPPayload(csrf, "", "", "", "")
+	payload["attribute_username"] = ""
+	req := NewRequestWithValues(t, "POST", "/admin/auths/new", payload)
+	session.MakeRequest(t, req, http.StatusSeeOther)
+
+	for _, u := range gitLDAPUsers {
+		req := NewRequest(t, "GET", "/admin/users?q="+u.UserName)
+		resp := session.MakeRequest(t, req, http.StatusOK)
+
+		htmlDoc := NewHTMLParser(t, resp.Body)
+
+		tr := htmlDoc.doc.Find("table.table tbody tr")
+		assert.True(t, tr.Length() == 0)
+	}
+
+	for _, u := range gitLDAPUsers {
+		req := NewRequestWithValues(t, "POST", "/user/login", map[string]string{
+			"_csrf":     csrf,
+			"user_name": u.UserName,
+			"password":  u.Password,
+		})
+		MakeRequest(t, req, http.StatusSeeOther)
+	}
+
+	auth.SyncExternalUsers(context.Background(), true)
+
+	authSource := unittest.AssertExistsAndLoadBean(t, &auth_model.Source{
+		Name: payload["name"],
+	})
+	unittest.AssertCount(t, &user_model.User{
+		LoginType:   auth_model.LDAP,
+		LoginSource: authSource.ID,
+	}, len(gitLDAPUsers))
+
+	for _, u := range gitLDAPUsers {
+		user := unittest.AssertExistsAndLoadBean(t, &user_model.User{
+			Name: u.UserName,
+		})
+		assert.True(t, user.IsActive)
+	}
+}
+
 func TestLDAPUserSyncWithGroupFilter(t *testing.T) {
 	if skipLDAPTests() {
 		t.Skip()