feat: support the empty string as a `default_value` for a variable

[dido18]

Description

It is not possible to set the default_value with an empty string in the brick configuration

For example, define the empty string for the PASSWORD in the brick_config.yaml

id: my:database
name: my personal database
description: "Connects to my database"
variables:
- name: PASSWORD
  description: the password
  default_value: ""

The expected usage in the app.yaml:

name: An app to use the database
icon: ☁️
description: connect to a database

bricks:
  - my:database:
      variables:
            PASSWORD:""

The result is that the PASSWORD with an empty string is used in the app.