fix(api): allow missing required variable when a brick is added

[dido18]

Motivation

Close #68

When a brick with required variables is added to an app created from scratch the operation fails.
This is because the FE (appLab) does not pass the required variables when the PUT /v1/apps/{appID}/bricks/{brickID} is called.
If the request body does not contain a value for a required variable, the operation fails.

Additionally, the issue is not present when an app is cloned.
This is because a cloned app already has the arduino:arduino_cloud brick configured in the app.yaml. In this scenario, the FE manages the required variables correctly by asking the users to fill in the required variables.

Change description

Do not fail with an error, but log a Warning and put the required variable as an empty string

Additional Notes

Reviewer checklist

• PR addresses a single concern.
• PR title and description are properly filled.
• Changes will be merged in main.
• Changes are covered by tests.
• Logging is meaningful in case of troubleshooting.

[dido18]

Test: create an app from scratch

1. Create an app from scratch

# app.yaml: The main configuration file for your Arduino App.
# This file describes the application's metadata and properties.

# The user-visible name of the application.
name: from scratch

# A brief description of what the application does.
description: ""

# The icon for the application can be an emoji or a short string.
icon: 😀

# A list of network ports that the application exposes.
# Example: [80, 443]
ports: []

# A list of bricks used by this application.
bricks: []

1. Add the Arduino Cloud brick

name: from scratch
description: ""
ports: []
bricks:
- arduino:arduino_cloud: {}
icon: 😀

1. Set the configuration variables

name: from scratch
description: ""
ports: []
bricks:
- arduino:arduino_cloud:
    variables:
      ARDUINO_DEVICE_ID: a
      ARDUINO_SECRET: b
icon: 😀