Not possible to dynamically pass nullable inputs from parent flow to subflow

[rybandrei2014]

Describe the issue

Hello team,

I have a flow that triggers subflow via Subflow task

id: TEST_NULL_ZPRACOVANI
namespace: csutrf
inputs:
- id: integerNull
  type: INT
  required: false
labels: []
tasks:
- id: TEST_NULL_1_subflow
  type: io.kestra.plugin.core.flow.Subflow
  namespace: csutrf
  flowId: TEST_NULL_1
  inputs:
    integerNull: "{{ inputs.integerNull }}"
concurrency:
  limit: 1
  behavior: CANCEL

id: TEST_NULL_1
namespace: csutrf
inputs:
- id: integerNull
  type: INT
  required: false
labels: []
tasks:
- id: log1
  type: io.kestra.plugin.core.log.Log
  message: "{{ inputs.integerNull }}"

I need to pass input variable from parent flow to the subflow. It works fine until the value is null, when I trigger parent flow with input that has flag required = false. The Subflow task throws the following error

integerNull: Invalid input for integerNull, For input string: "", but received `` 

I tried to modify the expression

"{{ inputs.integerNull }}" -> "{{ inputs.integerNull ?? 'null' }}" 

But it also throws an error

integerNull: Invalid input for integerNull, For input string: "null", but received null

Is it possible to somehow pass an input with a null value to the subflow?

P.S.: It's also not possible to achieve with TemplatedTask as it supports only Runnable tasks

Environment

• Kestra Version: 0.18.4

[loicmathieu]

This is not specific to subflow or INTEGER inputs, if a variable is null, our expression engine (Pebble) will return the empty string.
We may be able to do something about it for some input types but not for STRING.