Fix saving of empty multi select input

[nagno]

The original issue is described here: #6281

When you save an empty multi select input originally it sent an empty array, which was filtered out
by the magento2-base/lib/web/mage/utils/objects.js::flatten() method and it was not sent to the
backend for save. This fix changes the empty array to an empty string, which will be passed to
the backend and it saves it as empty value properly

[adprocas]

I'm not sure where you're seeing this issue come up, but I ended up fixing a very similar issue with a custom multiselect by adding this to the system.xml entry for it: <can_be_empty>1</can_be_empty>

[nagno]

@adprocas You can't create a system.xml entry for a product attribute. If you read the original github issue (#6281) you will see the exact steps to replicate it.

Come-on Magento, you can do better... The number of open PRs just increasing along with the number of open issuses...

[adprocas]

Oh, ok. Somehow I missed the original issue part of this. Either way, my comment may help others looking for how to solve the "problem" I was having.

[wigman]

This patch killed the attribute-set switcher (in 2.1.3) with the javascript error "Uncaught TypeError: Cannot read property 'length' of undefined".

To fix it I changed:
if (this.value().length == 0) {
to:
if (this.value() && this.value().length == 0) {

Many thanks for the patch though.

[nagno]

@wigman Thanks for the upgrade! :)
I tested it with 2.1.2 (EE) only and since that 2.1.3 came out. I really hope someone will check it from Magento and merge it before it will become completely outdated.

[TClilMouse]

The patch sometime doesn't save the input in CE 2.13.
Sometimes it does sometimes it doesn't. Not sure why.

[wigman]

@nagno you're welcome. I noticed you updated the normalize function, not sure this has the same effect as where I changed it: in the onUpdate function.

[nagno]

I reported this issue 6 months ago via the partner portal as well and now we finally got a working patch (the first attempt from them was not working), which is different than my solution, so I will close this pull request.
The solution is probably going to be included in one of the next Magento releases.