Fix issue 12894: Can't remove State is required for all countries

Author: Vasilii Burlacu

app/code/Magento/Directory/etc/adminhtml/system.xml

@@ -109,6 +109,7 @@
                 <field id="state_required" translate="label" type="multiselect" sortOrder="1" showInDefault="1" showInWebsite="0" showInStore="0">
                     <label>State is Required for</label>
                     <source_model>Magento\Directory\Model\Config\Source\Country</source_model>
+                    <can_be_empty>1</can_be_empty>
                 </field>
                 <field id="display_all" translate="label" type="select" sortOrder="8" showInDefault="1" showInWebsite="0" showInStore="0">
                     <label>Allow to Choose State if It is Optional for Country</label>