Bug: _UndefinedType is not JSON serializable

[skocan]

Description

When frontend_default_value is not specified (should use a default value <not-defined> in that case), there is an error that _UndefinedType is not JSON serializable.

Example Code

Can be also achieved with something like this:

from typing import Any
from pydantic import BaseModel, Field
from frinx.common.workflow.workflow import UNDEFINED

class WorkflowInputField(BaseModel):
    name: str
    frontend_default_value: Any = Field(UNDEFINED, alias='value')


testing_input = WorkflowInputField(name="test")
print(testing_input.model_dump_json())

>> pydantic_core._pydantic_core.PydanticSerializationError: Unable to serialize unknown type: <class 'frinx.common.workflow.workflow._UndefinedType'>

Python, Poetry & Packages Version

Virtualenv
Python:         3.11.6
Valid:          True

System
Platform:   linux
OS:         posix
Python:     3.11.6

...
frinx-python-sdk       0 0706f8e     Python SDK for Frinx Machine Workflow Manager
frinx-uniconfig-api    0.1.0 ccfdf04 Python SDK for Frinx Machine Workflow Manager
frinx-uniconfig-worker 1.0.0 576057a Conductor worker for Frinx Uniconfig
...
Poetry (version 1.7.1)