fix: null error when deleting subscriber

[patriciareinoso]

Description

fix #412

The imsis field of a new NS that has never had a subscriber associated is null
When a NS has already had a subscriber associated the field is []
We were assuming that this field was always [] so it failed trying to access a null value. In consequence the subscriber was not entirely deleted.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code.
• I have made corresponding changes to the documentation.
• I have added tests that validate the behaviour of the software.
• I validated that new and existing unit tests pass locally with my changes.
• Any dependent changes have been merged and published in downstream modules.