Empty field in yaml

You can use ~ or null.

You should read documentation of YAML and you can read Symfony Yaml Format as well

title:
    1: String
    2: String2
    3: ~

If you want an empty string, rather than a null value, you can use two single quotes.

title:
    1: String
    2: String2
    3: ''

According to YAML v1.2 spec:

10.3.2. Tag Resolution

Regular expression         Resolved to tag
null | Null | NULL | ~     tag:yaml.org,2002:null
/* Empty */                tag:yaml.org,2002:null

So putting null or ~ or omitting value produces the same result: tag:yaml.org,2002:null:

parent:
  key1:               # empty so "null", # is a comment!
  key2: ~             # also "null"
  key3: null          # "null" explicitly ))
  key4: !!null "null" # for the funs of "secondary tag handle: !!"
  key5: "null"        # sorry, it is a string or !!str if you like ((