Fix creating socket without item linked

[cconard96]

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• [x] I have read the CONTRIBUTING document.
• [x] I have performed a self-review of my code.
• [x] I have added tests that prove my fix is effective or that my feature works.

Description

Fixes #18202

Sockets are supposed to be allowed to exist without being connected to an item, but it was trying to save an empty string to the items_id column. Default selection for "Item" was an empty string instead of 0. Fix ensures that if the items_id field is an empty value, the itemtype and items_id values are not saved.

[trasher]

I've not been able to reproduce the initial issue. I've just created a Socket from the UI providing just a name; items_id was set to 0. From tests bootstrap data, it is set to null. Anyway, test do fail on current 10.0bugfixes branch without the fix, so it seems correct.