[Index Management][Mappings editor] Remove `fields` property if all multi-fields deleted

[yuliacech]

When a mapping field is using multi-fields, this data is stored in the fields property. When all multi-fields are deleted from a field, this property still contains an empty object. We need to update the serialization function for the mappings to remove the property if the field doesn't have any multi-fields.

How to reproduce

1. Create a component template with multi-fields

   PUT _component_template/template_1
   {
   "template": {
   "mappings": {
     "properties": {
       "command_line": {
         "fields": {
           "text": {
             "type": "match_only_text"
           }
         },
         "type": "wildcard"
       }
     }
   }
   }
   }

2. Navigate to Index Management -> Component templates and find the create component template
3. Navigate to the Mappings step in the wizard and remove the field text from the field command_line
4. Save the component template and open the details flyout
5. Check the Mappings tab

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