Symfony version(s) affected: v4.3.3

Description
A syntax error is ignored by the parser.

How to reproduce
This code:

paths:
  abc
  abc: def

is successfully parsed but it is not valid YAML.

Additional context
The following online parsers identify the error

• https://codebeautify.org/yaml-validator
• http://www.yamllint.com/

I have found several others that also identify the error correctly.