🐛 : non-editable mandatory variables with default value block stack creation

[juwit]

Describe the bug
non-editable mandatory variables with default value block stack creation

To Reproduce
Steps to reproduce the behavior:

1. Create a module
2. add a mandatory variable
3. add a default value to this variable
4. set this variable non-editable
5. create a stack from this module
6. try to "save and continue"
7. nothing happens, no error is show to the user
8. when using "development tools", we can see that the POST request to /api/stacks responds with a 400 error :

XHRPOST https://${url}/api/stacks
[HTTP/2.0 400 Bad Request 160ms]
{"message":"mandatory variables should not be blank"}

Expected behavior
When having a default value, a mandatory variable should always pass the validation

Screenshots
N/A

Desktop (please complete the following information):

• OS: Ubuntu 19.04
• Browser: Firefox 69
• Version: Gaia 1.2.0

Additional context
N/A