Mandatory multi-value field gets saved even when empty

It is possible to save a node with a mandatory term reference field even if we let the field empty.

How to reproduce:

• Create a vocabulary with some terms
• Create a Content Type with a term reference field to this vocabulary
• Make the field mandatory and choose the Autocomplete Deluxe widget and set the number of values to unlimited
• Create a node of that content type
• Try to save with the term reference field empty => This will not work, which is as expected
• Now, save it with some terms selected
• Edit the node again
• Try again to save with the term reference field empty => This works and this is not as expected

If we try the same with another widget, it works as expected, namely: It should be impossible to save a node with an empty term reference field if that field is mandatory.