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: ~
According to YAML v1.2 spec:
10.3.2. Tag Resolution
Regular expression Resolved to tagnull | 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 ((