Allow to unset birthday

Signed-off-by: marcin mikołajczak <git@mkljczk.pl>

test/pleroma/web/mastodon_api/update_credentials_test.exs

@@ -390,6 +390,20 @@ defmodule Pleroma.Web.MastodonAPI.UpdateCredentialsTest do
       assert user_data["source"]["pleroma"]["show_birthday"] == true
     end
 
+    test "unsets birth date", %{conn: conn} do
+      patch(conn, "/api/v1/accounts/update_credentials", %{
+        "birthday" => "2001-02-12"
+      })
+
+      res =
+        patch(conn, "/api/v1/accounts/update_credentials", %{
+          "birthday" => ""
+        })
+
+      assert user_data = json_response_and_validate_schema(res, 200)
+      assert user_data["pleroma"]["birthday"] == nil
+    end
+
     test "emojis in fields labels", %{conn: conn} do
       fields = [
         %{"name" => ":firefox:", "value" => "is best 2hu"},