Keep MultiValue.php synchronized with upstream

I have added a new patch to fixed these errors.

Checked both patches. Tested. Patch #5 is wrong: $element['#default_value'] is an empty array on initial form load, which changes behavior. (You get 0 empty elements.) It also has unwanted whitespace changes.

I'm committing this, but not closing the issue, because there is more to do here:

• The class' comment says:
  

   * Shamelessly copied from multivalue_form_element, on who we'll depend in 4.x.
   * (At the time of adding this, they seemed to have issues with D9
   * compatibility.)
  

• The changed code is not from the MultiValue.php currently commited in multivalue_form_element, but from a patch I made: 🐛 Don't add an empty element in some cases Needs review . This patch is now
  • outdated, because the change from this issue is not propagated into #3199298
  • outdated according to comment #3199298-8: Don't add an empty element in some cases → ($max should not occur anywhere in the code, so the patch likely should be rerolled)
  • still not going to be merged because it's missing tests
  • possibly complicated by the fact that I built my patch on another needs-review patch: if we add tests, we might need to split those two issues up again, for the reviewer.

This issue remains open to at least unify our own MultiValue.php with the patch, so that we don't lose any changes when we start depending on multivalue_form_element. (Or to track that we don't lose changes, in another way.)

This looks fixed from #9