[DX] Globally address the "Form API sets the value of unselected checkboxes to 0" issue.

[klonos]

I have noticed this pattern in numerous occasions over time, and I thought that we had an existing issue for it, but I am unable to find it.

Here are a couple of examples:

• https://github.com/backdrop/backdrop/blob/1.x/core/modules/admin_bar/admin_bar.inc#L746
• https://github.com/backdrop/backdrop/blob/1.x/core/modules/node/node.module#L2460
• https://github.com/backdrop/backdrop/blob/1.x/core/modules/system/system.admin.inc#L1646

Opening this ticket to see if there is anything we can do to address this globally, so that we do not have to be creating validation handler for it all the time.

[klonos]

...it seems that the longer this issue lingers, the more code debt we'll be adding in core and contrib. I would really like to understand why things have been implemented this way ...was it intentional behavior, in order to account for an actual use case; or was it simply an oversight at some point in the past?