### Bug Description Currently, the default charm part is added to the charmcraft project configuration using the Pydantic model validator `charmcraft.models.project.CharmcraftProject.preprocess_parts`. However, because Pydantic validators are executed in the order they are defined, and the `parts` field is defined in the parent class `craft_application.models.project.Project` while the `type` field is defined in the child class, the validator for `parts` field will always execute before validator for `type` field. This means the `type` attributes will never exist in the `values` parameter of `preprocess_parts` validator function. This causes the `parts` not to be added if the `parts` in the charmcraft project file are defined as `{}`. However, when the `parts` field is not defined in `charmcraft.yaml`, its value will be `None`, and the default value of `charmcraft.models.project.BasesCharm.parts` will take effect, setting the parts to `{"charm": {"plugin": "charm", "source": "."}}`. ### To Reproduce To reproduce this issue, create a `charmcraft.yaml` with the following parts. It will not execute the `charm` part when packed: ``` parts: {} ``` ### Environment Ubuntu 22.04 LTS ### charmcraft.yaml ```shell name: charm type: charm bases: - build-on: - name: ubuntu channel: "22.04" run-on: - name: ubuntu channel: "22.04" summary: A test charm. description: A test charm. # important parts: {} ``` ### Relevant log output ```shell n/a ```