Quick Fix for SubscripttonManagement servlet

[WilliamPetit]

Proposed changes

The UpdateSubscriptions() function was considering a subsciption with
missing status parameter as a 'subscription to update to unset'.

It mainly because the getTriBoxValue function return the same thing
whether the value of the parameter is 'null' or 'unset'.

Therefore, this fix simply make sure the parameter wasn't 'null' before
updating the subscription.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Feature modification (Fix or feature that will cause existing functionality to not work as expected)

Checklist

• Read the CONTRIBUTING doc
• Maintained the existing coding style and format
• Added tests that prove my fix is effective or that my feature works
• Added necessary documentation (if appropriate)
• Rebase and squash my commits. Submit one commit only, on top of the master branch.

Further comments

This hasn't been tested at all.

[fergaloy]

Unfortunately, this does not fix the problem. It behaves the same way as before, deleting subscriptions that are in tabs that have not been visited.