Isset

Available configuration keys: 

• negate: (optional) Whether the 'isset' condition should be negated. Defaults to FALSE. You can also negate the 'isset' plugin by using 'not:isset' as the plugin id.
• source: (optional) Property or array of properties on which to evaluate the condition. If not set, the condition will be evaluated on the source passed to the ::evaluate() method, typically the source of the process plugin that is using this condition.

Examples:

1. Skip a row if skip_me_please is set. 

process:
  skip_requested:
    plugin: skip_on_condition
    condition: isset
    source: skip_me_please
    method: row
    message: 'skip was requested'

 2. Re-create the core null_coalesce process plugin

isset and is_null are essentially opposites.

process:
  some_not_null_value:
    plugin: first_meeting_condition
    condition: isset
    source:
      - some_value
      - another_value
      - yet_another
    default_value: 'If all else fails...'