Entering non-existing term as default to taxonomy reference field leads to wired data in field.instance config file.

Problem/Motivation

Steps to reproduce:

1. After installing site with standard profile, make sure you have default_value:{} in sites\default\files\config_[hash]\active\field.instance.node.article.field_tags.yml
2. Go to admin/structure/types/manage/article/fields
3. Edit Tags
4. on "Default value" enter some free text and click "Save settings". (make sure widget type is "Autocomplete term widget (tagging)")
5. Now check the same file in step 1 and you can see default_value as below:
   
       tid: '0'
    entity:
      tid: ''
      uuid: b4061cc9-39c5-448a-90b9-5318f9af56db
      vid: category
      name: freetext
      description: ''
      format: ''
      weight: '0'
      parent:
        - '0'
      langcode: und
      "\0*\0entityType": taxonomy_term
      "\0*\0enforceIsNew": ''
      "\0*\0newRevision": '0'
      "\0*\0isDefaultRevision": '1'

Remaining tasks

Discuss and issue patch, if it is a bug.