Schema validator doesn't recognize Null literals

[imroc]

Describe the bug

Schema validator output: Incorrect type, Expected string

Field schema:

{
  "description": "Time is a wrapper around time.Time which supports correct marshaling to YAML and JSON.  Wrappers are provided for many of the factory methods that the time package offers.",
  "format": "date-time",
  "type": "string"
}

Just like #118 , bug came back again?

yaml-language-server version uses the latest release 1.15.0