studio: in db, delete deck attributes instead of keep null values

[peterpeterparker]

Affected part of DeckDeckGo

• [X ] Studio

Feature Description

A technical minor task: instead of keeping null values for background and attributes of the deck in db in case these information would be delete, remove (delete) the attributes from the doc.

Like for the background:

// @ts-ignore
 currentDeck.data.background = background && background !== undefined && background !== '' ? background : firebase.firestore.FieldValue.delete();